Let’s break down the creation of this Peanut Butter Chocolate Layer Cake. First, prepare for the cake layers. Preheat your oven to 350°F (176°C). Grease and flour three 8-inch round cake pans, and line the bottoms with parchment paper circles. This triple preparation ensures your moist cakes won’t stick.

In a large mixer bowl (or just a large bowl if using a hand mixer or whisking by hand), combine all the dry cake ingredients: all-purpose flour, granulated sugar, natural unsweetened cocoa powder, baking soda, baking powder, and salt. Whisk them together thoroughly to ensure everything is evenly distributed, especially the leavening agents. Breaking up any cocoa lumps here is helpful. Set this bowl aside.

In a separate medium-sized bowl, whisk together the wet ingredients (except the hot water): milk, vegetable oil, vanilla extract, and the large eggs. Make sure the eggs are fully incorporated. Pour these combined wet ingredients into the bowl containing the dry ingredients.

Beat the mixture on medium-low speed (with your stand mixer or hand mixer) just until everything is combined and no large streaks of dry ingredients remain. Be careful not to overmix at this stage. Scrape down the sides and bottom of the bowl to ensure all ingredients are incorporated. The batter will be relatively thick at this point.

Now, carefully pour the hot water (or hot coffee, if using) into the batter. Mix on low speed until the water is fully incorporated and the batter is smooth and quite thin. This thin batter is characteristic of oil-based chocolate cakes and contributes to their moistness. Scrape the bowl one last time.

Divide the thin batter evenly among the three prepared cake pans. Using a kitchen scale is the best way to ensure even layers, which bake more uniformly and stack better. Bake in the preheated 350°F (176°C) oven for approximately 22-25 minutes. The cakes are done when a wooden toothpick inserted into the center comes out with a few moist crumbs attached (not wet batter). Overbaking will lead to a dry cake.

Let the cakes cool in their pans on wire racks for just 2-3 minutes. Then, carefully run a thin knife or offset spatula around the edges and invert the cakes onto the wire racks. Peel off the parchment paper and let them cool completely before attempting to level or frost them. Warm cakes and frosting are a recipe for disaster!

While the cakes cool, make the peanut butter frosting. In your large mixer bowl (preferably with the paddle attachment if using a stand mixer), beat the room temperature salted butter and creamy peanut butter together until well combined and smooth. This ensures there are no lumps of butter or peanut butter in the final frosting.

Gradually add the powdered sugar. Start by adding about half of the powdered sugar (around 4.5 cups) and mix on low speed until it’s incorporated, then increase the speed to medium and beat until smooth. Scrape down the bowl. Add about 3 tablespoons of the water or milk and mix until smooth. This helps loosen the mixture before adding the rest of the sugar.

Add the remaining powdered sugar (the other 4.5 cups) and mix again on low speed until incorporated, then beat on medium speed until smooth. Add the remaining water or milk, one tablespoon at a time, mixing until smooth after each addition. Stop adding liquid once the frosting reaches your desired spreading consistency – it should be thick but easily spreadable, not stiff or runny. Beat for another minute or two until light and fluffy. Don’t over-add liquid! Set aside about 1-1.5 cups of frosting for the final swirls on top, if desired.

Now, let’s build the cake! Once the cake layers are completely cool, use a long serrated knife to level the tops, removing any domes so you have flat, even surfaces. This is crucial for a stable cake that doesn’t lean. Place the first leveled cake layer onto your serving plate or a cardboard cake circle.

Top this first layer with about 1 cup of the peanut butter frosting, spreading it into an even layer almost to the edge using your offset spatula. Sprinkle about half of the chopped Reese’s (3 cups) evenly over the frosting layer. Gently press them into the frosting. Spread a very thin layer of additional frosting over the Reese’s – this acts like glue for the next cake layer.

Carefully place the second leveled cake layer on top. Repeat the process: top with about 1 cup of frosting, spread evenly, sprinkle the remaining half of the chopped Reese’s (the other 3 cups), press them in gently, and add that thin “glue” layer of frosting over the top. Add the third and final leveled cake layer, placing it top-side down for the flattest possible surface on top of the cake.

Apply a thin layer of frosting all over the sides and top of the cake – this is the “crumb coat.” It traps any loose crumbs and provides a smooth base for the final frosting layer. Use your offset spatula to make it as smooth as possible, but don’t worry about perfection here. Chill the crumb-coated cake in the refrigerator for about 15-20 minutes to firm up.

Apply the remaining frosting (except the portion set aside for piping) to the sides and top of the chilled cake. Use your offset spatula and bench scraper (if using) to create a smooth finish. The original recipe suggests creating vertical stripes using the offset spatula for texture, which means perfect smoothness isn’t strictly necessary if you choose that design. Set the frosted cake aside while you make the ganache.

For the chocolate ganache, place the semi-sweet chocolate chips (or chopped chocolate) in a medium heatproof bowl. Heat the heavy whipping cream in a separate small saucepan or in the microwave just until it begins to gently boil or simmer around the edges. Immediately pour the hot cream over the chocolate chips. Let the mixture sit undisturbed for 2-3 minutes to allow the heat from the cream to melt the chocolate.

After letting it sit, gently whisk the mixture, starting from the center and working your way out, until the chocolate is completely melted and the ganache is smooth, glossy, and uniform. Avoid over-whisking, which can incorporate too much air. Let the ganache cool slightly for about 5-10 minutes; it should be pourable but slightly thickened, not hot or watery. If it’s too thin, let it cool a bit longer. If it thickens too much, you can gently reheat it for a few seconds.

Now for the signature drip! Using a squeeze bottle or a spoon, carefully drizzle the slightly cooled ganache around the top edge of the chilled cake, letting it drip down the sides. Control the length of the drips by how much ganache you apply at the edge. Once the drips are done, pour the remaining ganache onto the center of the cake top and use your small offset spatula to gently spread it evenly to meet the dripped edge.

Let the ganache set for about 10-15 minutes at room temperature or briefly in the fridge until it’s no longer wet but still slightly soft. Fit a piping bag with your chosen large star tip (like Ateco 844, Wilton 1M, or 2D) and fill it with the frosting you set aside earlier. Pipe decorative swirls around the top edge of the cake, on top of the ganache. Garnish the swirls with the halved Reese’s Peanut Butter Cups and sprinkle some crumbled Reese’s in the center or around the base, if desired.

Refrigerate the finished cake for at least 30 minutes to allow the frosting and ganache to fully set before slicing. For the best flavor and texture, serve the cake at room temperature.