Preheat the oven to 350°F. Generously grease two 8-inch wide x 3-inch deep cake pans and line with parchment paper rounds.

In a large mixing bowl, whisk together the flour, sugar, cocoa, baking soda, baking powder, and salt.

In a medium bowl, whisk together the sour cream, vegetable oil, eggs, vanilla, and beer.

Add the wet ingredients into the dry ingredients and stir with a spatula until just combined, being careful not to overmix.

Divide the batter between the prepared pans and bake for 35 to 40 minutes, or until a cake tester comes out clean.

Cool the cakes in their pans for 30 minutes before carefully turning them out onto a cooling rack or wire rack to cool completely.

In a large bowl, use an electric mixer or hand mixer to beat the butter until light and fluffy.

Gradually add the sifted powdered sugar, then the Irish cream. Beat the mixture on high speed until very light, fluffy, and smooth, about 3 minutes.

Place one cake layer on a cake stand or serving plate. Frost the top of the cake generously.

Top with the other cake layer, flat side up.

With an offset spatula, spread a very thin layer of frosting all over the cake. This layer is a crumb coat.

Spread the remaining frosting all over the cake. If desired, focus the frosting on the top to maintain the 'naked' look.

Place the chopped chocolate in a heatproof bowl. In a small pan, bring the heavy cream to a simmer then immediately remove from heat and pour over chocolate.

Allow to cool until it has thickened but is still pourable, about 10 minutes.

Carefully pour some of the ganache over the top of the chilled cake. Use an offset spatula to quickly smooth the ganache over the top of the cake before it sets.

Before the ganache fully sets, add sprinkles, chocolate curls, or any other decor you like.