Let’s build this decadent Peanut Butter Chocolate Layer Cake together! First, prepare the oven and pans for the chocolate cake layers. Preheat your oven to 350°F (176°C). Thoroughly prepare three 8-inch round cake pans using non-stick baking spray and placing parchment paper circles in the bottoms. Set aside.

Make the moist chocolate cake batter. In a large mixing bowl, add the all-purpose flour, granulated sugar, natural unsweetened cocoa powder, baking soda, baking powder, and salt. Whisk these dry ingredients together thoroughly to combine and break up any cocoa lumps. Set aside. In a separate medium-sized bowl, whisk together the room temperature milk, vegetable oil, vanilla extract, and room temperature eggs until well combined.

Pour the combined wet ingredients into the bowl containing the dry ingredients. Using an electric mixer on low speed or a large whisk, beat just until the ingredients are well combined and moistened. Scrape down the sides and bottom of the bowl.

Carefully and slowly pour the hot water (or hot coffee) into the batter while mixing on low speed. Continue mixing only until the hot liquid is fully incorporated and the batter is smooth. It will be a thin batter – this is expected and contributes to the moistness. Ensure everything is well combined with a final scrape of the bowl.

Divide the thin batter evenly among the three prepared cake pans. Using a kitchen scale provides the most accuracy for even layers. Bake in the preheated oven for 22-25 minutes. The cakes are done when a wooden toothpick inserted into the center comes out with only a few moist crumbs attached (not wet batter). Be careful not to overbake.

Remove the cakes from the oven and allow them to 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 frosting.

While the cakes cool, make the peanut butter frosting. In a large mixer bowl (preferably with the paddle attachment if using a stand mixer), beat the room temperature salted butter and the creamy peanut butter together on medium-high speed until well combined, smooth, and fluffy (about 3-4 minutes). Scrape down the bowl.

Turn the mixer speed to low. Gradually add about half of the powdered sugar (around 4.5 cups) and mix until incorporated, then increase speed slightly and beat until smooth. Add 3 tablespoons of the water or milk and mix until smooth. Add the remaining powdered sugar (the other 4.5 cups), mixing on low first, then beating until smooth. Add the remaining 3-4 tablespoons of water or milk, one tablespoon at a time, beating well after each addition, until the frosting reaches a smooth, creamy, and easily spreadable consistency. Be careful not to add too much liquid and make it too thin. Beat for another minute until light and fluffy.

Assemble the cake! Once the cake layers are completely cool, use a large serrated knife to level the tops, removing any domes for flat, stable stacking. Place the first leveled cake layer onto your serving plate or cardboard cake circle. Top 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 (3 chopped cups) of the chopped Reese’s evenly over the frosting layer. Gently press them into the frosting. Spread a very thin layer of additional frosting just over the Reese’s – this helps the next cake layer adhere securely.

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 (other 3 chopped cups) of the chopped Reese’s, press gently, and add that thin “glue” layer of frosting over the candies. Add the third and final leveled cake layer, placing it top-side down for the flattest possible surface on top.

Apply a thin layer of the peanut butter frosting all over the sides and top of the cake – the “crumb coat.” This traps loose crumbs for a cleaner final finish. Chill the crumb-coated cake in the refrigerator for about 15-20 minutes to firm up.

Apply the remaining peanut butter frosting (reserving some for piping, if desired) generously to the top and sides of the chilled cake. Smooth the top with your offset spatula. The recipe notes suggest potentially creating stripes or textures on the sides using the offset spatula, or you can aim for a smooth finish using an offset spatula and potentially a bench scraper. Set the frosted cake aside (chilling it again is recommended before adding the ganache drip).

Make the chocolate ganache. Place the semi-sweet chocolate chips in a medium heatproof bowl. Heat the heavy whipping cream in a small saucepan or in the microwave until it just begins to boil or simmer vigorously 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. Let the ganache cool slightly at room temperature for about 10-15 minutes, or until it’s thickened enough to create drips but is still pourable (not hot).

Apply the ganache drip to the chilled cake. Using a squeeze bottle or a spoon, carefully drizzle the slightly cooled ganache around the top edge of the cake, letting it drip down the sides. Control the length and frequency of the drips as desired. Pour remaining ganache onto the center top and gently spread it to meet the dripped edge with a small offset spatula.

Allow the ganache to set for about 10-15 minutes (chilling helps). If desired, transfer reserved peanut butter frosting to a piping bag fitted with a large closed star tip (like Ateco 844, Wilton 1M, or 2D). Pipe decorative swirls or rosettes around the top edge of the cake on the ganache. Garnish lavishly with the halved or crumbled Reese’s Peanut Butter Cups. Add an optional final drizzle of salted caramel sauce over everything, if you like. Refrigerate the cake until ready to serve, but remember to serve at room temperature.