Preheat oven, Grease and lightly flour two baking pans.

In a saucepan melt butter and chocolate over low heat, stirring often; set aside. In a large mixer bowl combine eggs, sugar and vanilla.
Set bowl over (not touching) 1 to 2 inches of hot (not boiling) water in a large saucepan.
Heat over low heat, stirring occasionally, about 10 minutes or till lukewarm. Remove from heat; remove bowl from saucepan.
Beat with an electric mixer on high speed about 15 minutes or till nearly tripled in volume. Gently fold in flour, 1/3 cup at a time. Gradually fold in chocolate mixture. Spread evenly in prepared pans.

Bake until a wooden toothpick inserted near the center comes out clean. Cool 10 minutes on wire racks. Remove from pans; cool completely. Prepare Espresso Buttercream. Fill and frost cake with Espresso Buttercream. Pipe Buttercream around the edge of cake.

Buttercream

Beat egg yolk with electric mixer till thick and lemon colored; set aside. In a medium saucepan combine sugar, water, and coffee powder, bring to boiling, stirring till dissolved. Cook over medium high heat. Stir constantly, till mixture reaches soft-ball stage (236F). Quickly pour the hot mixture in a steady stream over yolks, beating constantly on high speed. Continue beating till mixture is thick and smooth. Cool 15 minutes. Meanwhile, beat the unsalted butter till light and fluffy. Beat butter, 1 tablespoon at at time, into cooled yolk mixture. Cover and chill for 30 minutes or until stiff enough to spread. Stir semisweet chocolate pieces into 1/2 cup of the Buttercream.