Line two greased 9-in. round baking pans with waxed paper and grease the paper; set aside. In a large bowl, beat butter and brown sugar until crumbly. Beat in egg yolks and vanilla. Stir in chocolate and coffee. Combine flour, salt and baking soda; gradually add to chocolate mixture alternately with buttermilk (batter will be thick).
Pour into prepared pans. Bake at 350° for 25-30 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes.
Meanwhile, in a small saucepan, bring sugar, milk and butter to a gentle boil; cook and stir for 2 minutes or until sugar is dissolved. Remove from the heat; stir in extract.
Invert cakes onto wire racks; drizzle with glaze. Cool completely. In a microwave, melt chocolate chips and milk; stir until smooth. Refrigerate until spreadable, stirring occasionally.
Place one cake on a serving plate; spread with filling. Top with remaining cake. Spread whipped topping over top and sides of cake. Store in the refrigerator. Garnish with chocolate curls. Yield: 12 servings.