Grease and flour three 9-in. round baking pans; set aside.
In a large saucepan, combine the beer, butter and chocolate. Cook and stir over medium-low heat until butter and chocolate are melted. Remove from the heat; whisk in milk powder and cocoa. Transfer to a large bowl. Let stand for 15 minutes.
Add sour cream and eggs to chocolate mixture; beat until well blended. Combine the sugar, flour, baking soda and salt; gradually beat into chocolate mixture until blended. Pour batter into prepared pans.
Bake at 350° for 32-36 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.
In a large heavy saucepan, bring sugar and liqueur to a gentle boil over medium heat; cook until sugar is dissolved. Remove from the heat. Add a small amount of hot mixture to egg yolks; return all to the pan, stirring constantly. Cook 2 minutes longer or until mixture thickens, stirring constantly. Remove from the heat; stir in vanilla. Cool to room temperature.
In a large bowl with a whisk attachment, cream butter until fluffy, about 5 minutes. Gradually beat in sugar mixture. Add milk powder; beat until fluffy, about 5 minutes. If necessary, refrigerate until frosting reaches spreading consistency.
Place bottom cake layer on a serving plate; spread with 1 cup frosting. Repeat layers. Top with remaining cake layer. Spread remaining frosting over top and sides of cake. Refrigerate for at least 1 hour before serving. Yield: 16 servings.