Let’s break down the process for this delicious cake:

Make the Cake:

1. Preheat and Prepare Pans: Preheat the oven to 350°F (175°C). Generously grease two 8-inch wide x 3-inch deep cake pans and line with parchment paper rounds.
2. Combine Dry Ingredients: In a large mixing bowl, whisk together the flour, sugar, cocoa, baking soda, baking powder, and salt.
3. Combine Wet Ingredients: In a medium bowl, whisk together the sour cream (or yogurt), vegetable oil, eggs, vanilla, and Guinness beer.
4. Combine Wet and Dry: Add the wet ingredients into the dry ingredients and stir with a spatula until just combined. Be careful not to overmix.
5. Bake: Divide the batter evenly between the prepared pans. Bake for 35 to 40 minutes, or until a cake tester inserted into the center comes out clean.
6. Cool: Cool the cakes in their pans for 30 minutes before carefully turning them out onto a cooling rack to cool completely. Freezing the cooled cake layers briefly can make frosting easier.

Make the Buttercream:

1. Beat Butter: In a large bowl, use an electric mixer to beat the butter (at cool room temperature) until light and fluffy.
2. Add Sugar and Irish Cream: Gradually add the sifted powdered sugar, alternating with the Irish cream liqueur. Start on low speed, then increase to high.
3. Beat Until Fluffy: Beat the mixture on high speed until very light, fluffy, and smooth, about 3 minutes. If the frosting is too thick, add a splash more Irish cream. If it’s too thin, add more sifted powdered sugar.

Assemble the Cake:

1. First Layer: Place one cooled cake layer on a cake stand or serving plate. Frost the top generously with a portion of the Irish buttercream.
2. Second Layer: Top with the other cake layer, placing it flat side up (top side down) for an even surface.
3. Crumb Coat: Using an offset spatula, spread a very thin layer of frosting all over the cake (top and sides). This “crumb coat” seals in any loose crumbs.
4. Chill Crumb Coat: Chill the cake in the fridge or freezer until the crumb coat is firm to the touch (about 15-30 minutes).
5. Final Frosting: Spread the remaining frosting smoothly all over the cake. If desired, focus the frosting on the top and create a “naked” or “semi-naked” look on the sides as shown in some photos. Refrigerate while you prepare the chocolate drip.

Make and Apply Chocolate Drip:

1. Prepare Ganache: Place the finely chopped chocolate in a heatproof bowl. In a small pan, bring the heavy cream just to a simmer, then immediately remove from heat and pour over the chocolate. You can also heat the cream in the microwave. Let stand for a few minutes.
2. Stir Ganache: Stir the mixture gently until the chocolate is completely melted and the ganache is smooth.
3. Cool Ganache: Allow the ganache to cool until it has thickened slightly but is still pourable (about 10 minutes). Don’t place it in the fridge. Test the consistency by letting a drip run down the side of a cool glass. If it runs too fast, let it cool longer. If too thick, gently warm for 5-10 seconds.
4. Apply Drip: Carefully pour some of the ganache onto the center of the chilled cake. Use an offset spatula to quickly smooth it over the top, letting it just reach the edge. Use a squeeze bottle or a spoon to gently guide drips down the sides of the cake.
5. Add Garnishes: Before the ganache fully sets, add sprinkles, chocolate curls, or any other decorations you like.
6. Store This cake can be stored, covered with a cake keeper, at a cool, dry room temperature for 2-3 days.

Take your time with assembly for the best results!