Preheat oven to 350F.

Line bottoms of 2 9-inch round cake pans with waxed paper. Grease paper and sides of pan. Dust with flour.

Mix flour, cocoa, baking soda, and salt. In another bowl, beat butter sugar, and brown sugar at medium speed until light and fluffy. Add eggs, one at a time, beating well after each addition. Add vanilla. At low speed, alternately beat flour mixture and buttermilk into butter mixture just until blended. Divide batter between prepared pans.

Bake cakes until a toothpick inserted in center comes out clean, 25 to 30 minutes. Transfer pans to wire racks to cool for ten minutes. Turn out onto racks and remove paper. Turn layers topside up and cool completely.

To prepare frosting, beat butter and powdered sugar at medium speed until light and fluffy. Add melted chocolate and vanilla; continue beating until shiny and smooth.

Place one cake layer on a serving plate; spread with frosting. Top with remaining cake layer; spread frosting on top and sides of cake. Let cake stand for at least 30 minutes before sprinkling with chocolate shavings and slicing.