1. Prepare Oven and Pans:

Preheat oven to 350°F (175°C). Grease three 8-inch round cake pans, line the bottoms with parchment paper rounds, and grease the parchment. Dust lightly with flour or cocoa powder.

2. Combine Dry Cake Ingredients:

In a large bowl (stand mixer bowl preferred), whisk together the flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt until well combined.

3. Combine Wet Cake Ingredients:

In a separate medium bowl, whisk together the room temperature sour cream (or yogurt), vegetable oil, room temperature eggs, and vanilla extract.

4. Combine Batter:

Add the wet ingredients to the dry ingredients in the mixer bowl. Beat on low speed, then medium speed until just combined.

With the mixer on low speed, slowly pour in the Guinness beer. Mix just until smooth and fully incorporated. Do not overmix. The batter may be somewhat thin.

5. Bake the Cake Layers:

Divide the batter evenly among the three prepared cake pans.

Bake for 25-30 minutes (Self-correction: Recipe directions say 30-35 mins for potentially different pan sizes. Let’s use that range). Revised: Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

6. Cool the Cakes:

Let the cakes cool in the pans on wire racks for 10 minutes.

Then, carefully invert the cakes onto the cooling racks, remove the parchment paper, and let them cool completely.

7. Make the Irish Buttercream:

While the cakes cool, make the frosting. In the clean bowl of your stand mixer (paddle attachment), beat the cool room temperature butter on medium-high speed until smooth and creamy (about 1-2 minutes).

Reduce speed to low. Gradually add the sifted powdered sugar, about 1-2 cups at a time, mixing until incorporated.

Add the Irish cream liqueur and salt (if using). Mix on low until combined.

Increase speed to medium-high and beat for 3-5 minutes until the buttercream is light, fluffy, and smooth. If too stiff, add a tiny splash more Irish cream or milk; if too soft, beat in a little more sifted powdered sugar.

8. Assemble the Cake:

Level the tops of the completely cooled cake layers if necessary.

Place the first cake layer on your serving plate or cake stand. Spread a generous layer of Irish buttercream evenly over the top (about 1 cup).

Place the second cake layer on top. Spread with another generous layer of frosting.

Place the final cake layer on top (upside down for a flat top).

Apply a thin crumb coat of frosting over the entire cake. Chill for 20-30 minutes until firm.

9. Final Frosting:

Frost the entire chilled cake smoothly with the remaining Irish buttercream using an offset spatula and/or bench scraper.

Chill again for at least 30 minutes to firm the frosting before adding the ganache drip.

10. Make and Apply Ganache Drip:

Place the finely chopped semi-sweet or bittersweet chocolate in a heatproof bowl.

Heat the heavy cream in a small saucepan or microwave until just simmering (do not boil).

Pour the hot cream over the chocolate. Let sit for 2-5 minutes without stirring.

Gently whisk until the ganache is completely smooth and glossy. Let it cool slightly until it’s thickened but still pourable (test consistency on a cold plate).

Using a squeeze bottle or spoon, carefully apply drips around the top edge of the cold, frosted cake.

Pour remaining slightly cooled ganache onto the center top of the cake and gently spread to the edges.

11. Decorate and Serve:

Let the ganache set slightly (can chill briefly).

Decorate with sprinkles or chocolate curls, if desired.

For best flavor and texture, let the cake sit at room temperature for 30-60 minutes before slicing and serving. Store leftovers covered in the refrigerator.