Let’s bake this incredible Guinness Chocolate Cake! First, prepare your oven and pans. Preheat oven to 350°F (175°C). Grease three 8-inch round cake pans (or two 9-inch pans). Line the bottoms with parchment paper circles and grease the parchment. Set aside. Ensure key ingredients (sour cream/yogurt, eggs, Guinness, butter for frosting, Irish Cream) are at room temperature as specified. Letting Guinness sit out briefly can help it go slightly flat, which is sometimes preferred for baking.

Prepare the cake batter using a simple method. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, sifted cocoa powder, baking soda, baking powder, and fine salt. Make sure they are thoroughly combined and free of lumps.

In a separate medium bowl, whisk together the room temperature sour cream (or yogurt), vegetable oil, room temperature eggs, and vanilla extract until smooth. Pour this wet mixture into the large bowl containing the dry ingredients. Using an electric mixer on low speed or a large whisk/spatula, begin mixing the wet and dry ingredients together.

Slowly pour in the room temperature Guinness beer while mixing on low speed. Continue to mix just until the ingredients are combined and the batter is smooth. Scrape the bottom and sides of the bowl to ensure no dry pockets remain, but avoid overmixing, which can make the cake tough. The batter might be relatively thin.

Divide the finished batter evenly among the prepared cake pans. Gently tap the pans on the countertop to release any large air bubbles. Bake for approximately 25-35 minutes (time will vary depending on pan size – 8-inch layers will likely bake faster than 9-inch). The cakes are done when a wooden toothpick inserted into the center comes out clean or with a few moist crumbs attached.

Remove the cakes from the oven and let them cool in their pans on wire racks for about 10 minutes. Then, carefully run a thin knife or 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 Irish Buttercream. Ensure your butter is at a cool room temperature – softened and pliable but not greasy or melty. In a large bowl using an electric mixer (stand mixer with paddle or handheld), beat the butter on medium-high speed for 3-5 minutes until it’s very pale, light, and fluffy. Scrape down the bowl.

Turn the mixer speed down to low. Gradually add the sifted powdered sugar, about 1-2 cups at a time, mixing until just incorporated after each addition. Once all the sugar is added, scrape the bowl again. Add the room temperature Irish Cream liqueur. Start mixing on low speed, then increase to medium-high and beat for another 2-3 minutes until the liqueur is fully incorporated and the frosting is smooth, light, and fluffy. If the frosting seems too stiff, you can beat in a splash more Irish Cream or regular cream/milk (1 tsp at a time). If it seems too soft or tries to curdle (unlikely if ingredients are right temp and liqueur added gradually), try chilling briefly and re-whipping.

Prepare the Chocolate Ganache Drip. Finely chop the semi-sweet or bittersweet chocolate and place it in a heatproof bowl. Heat the heavy cream in a small saucepan over medium heat just until it simmers (bubbles around edge). Pour the hot cream directly over the chopped chocolate. Let it sit undisturbed for 3-5 minutes. Gently whisk the mixture, starting from the center, until the ganache is completely smooth and glossy. Let it cool at room temperature until it’s lukewarm or slightly warmer than room temp and has thickened to a consistency that will drip nicely down the side of a cold cake without running straight to the bottom.

Assemble the cake! Level the tops of the completely cooled cake layers with a serrated knife if desired. Place the first cake layer onto your serving plate or cake stand. Spread a generous layer of the Irish Buttercream evenly over the top (about 1 cup).

Carefully place the second cake layer on top. Add another generous layer of frosting. If using three layers, place the third layer on top. Use the remaining Irish Buttercream to frost the top and sides of the entire cake. Use an offset spatula to smooth the frosting as desired. Chill the frosted cake for at least 15-20 minutes to firm up the buttercream before adding the drip.

Carefully apply the ganache drip. Using a spoon or squeeze bottle, apply the slightly cooled ganache around the top edge of the chilled cake, letting it drip down the sides. Control the drip length by the amount applied and the ganache temperature/consistency. Pour any remaining ganache onto the center top if desired and spread gently.

Let the ganache set for a few minutes (chilling speeds this up). Garnish with optional sprinkles or chocolate curls/shavings just before the ganache sets completely. Store the cake in the refrigerator, but serve at cool room temperature.