Make the Cake: Preheat the oven to 350°F (175°C). Grease two 8-inch wide x 3-inch deep cake pans and line with parchment paper.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt.
In a medium bowl, whisk the sour cream, vegetable oil, eggs, vanilla, and Guinness beer.
Add the wet ingredients to the dry ingredients and stir with a spatula until just combined. Do not overmix.
Divide the batter between the prepared pans and bake for 35 to 40 minutes, or until a toothpick comes out clean.
Let the cakes cool in their pans for 30 minutes, then carefully transfer to a wire rack to cool completely. Freeze the cakes while you prepare the buttercream.
Make the Buttercream: Using an electric mixer, beat the butter until light and fluffy. Gradually add sifted powdered sugar, followed by the Irish cream. Beat on high speed for about 3 minutes, until smooth and fluffy.
Assemble the Cake: Place one cake layer on a serving plate and frost the top generously. Add the second layer, flat side up. Apply a thin crumb coat of frosting over the entire cake. Freeze until firm.
Spread the remaining buttercream over the cake. For a ‘naked’ cake, focus the frosting on the top and lightly coat the sides. Chill the cake while you prepare the chocolate drip.
Make the Chocolate Drip: In a heatproof bowl, place the chopped chocolate. Heat the heavy cream to a simmer and pour over the chocolate. Let it sit for a few minutes, then stir until smooth.
Let the chocolate cool for 10 minutes until thickened but pourable. Drizzle the ganache over the top of the chilled cake, allowing it to drip down the sides. Decorate with sprinkles or chocolate curls if desired.