Preheat oven to 350°F. Butter two 8-inch round cake pans with 2-inch-high sides. Line with parchment paper. Butter paper and sides of the pan.
Add the stout (or coffee) and butter to a heavy large saucepan over medium heat. Bring to a simmer and remove from the heat. Add the cocoa powder and whisk until mixture is smooth. Cool slightly.
In a large bowl add your flour, sugar, baking soda, and salt whisk to blend. Set aside.
Add the eggs and sour cream to a large bowl. Beat with an electric or stand mixer (use paddle attachment) to blend. Add the cooled stout-chocolate mixture to the egg mixture and beat on low, just to combine.
Add flour mixture gradually, beating on slow. Using a rubber spatula, fold batter until completely combined. Divide batter equally among prepared pans. Allow the cakes to rest for 10 minutes before baking.
Bake cakes until toothpick inserted into the center of the cakes comes out clean, about 35 minutes. Transfer cakes to rack; cool 10 minutes. Turn cakes out onto a rack and cool completely.
Irish Buttercream Frosting
With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes.
Add confectioners’ sugar, Baileys, and salt. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2 full minutes. Add more confectioners’ sugar if frosting is too thin or another tablespoon of Baileys if frosting is too thick.
Whiskey Glaze
In a small saucepan combine the water and sugar. Heat over medium heat until the sugar dissolves stirring constantly. Remove from heat and stir in the whiskey. Cool before assembling the cake.
Cake Assembly
While your cake is baking and cooling make your glaze and frosting. Dot the cake plate with frosting and place 1 cake layer on the plate. If the cake is domed use a bread knife to horizontally trim the to make the top even. Use a pastry brush to spread on the whiskey glaze.
Using a small offset spatula, evenly cover the top of the first layer with 1 cup of buttercream. Spread the buttercream so it extends beyond the edges of the cake. Trim the second layer and gently place it on top of the first tier. Brush with the whiskey glaze.
Use the remaining buttercream to frost the sides and the outside of the cake.
Notes
Add 1 teaspoon xanthan gum if your flour blend does not already contain
A kitchen scale can be used to ensure each cake pan has an equal amount of batter
You may need more or less confectioners sugar for the buttercream depending on the brand of butter and the temperature and humidity of your kitchen