Preheat oven to 350 oF Line bottoms of two 9 inch round cake pans with waxed paper. Grease paper and sides of pans. Dust with flour.
Mix flour and cocoa, in another big bowl, beat 1/2 cup of butter, granulated sugar and brown sugar at medium speed until light and fluffy. Add eggs 1 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 equally between prepared pans.
Bake cakes for 25 - 30 minutes or until a toothpick inserted in center comes out clean.
Transfer pans to wire racks to cool for 10 minutes. Turn out onto racks. Remove paper. Turn layers top-side up and cool completely.
To prepare frosting, beat butter and confectioners sugar at medium speed until light and fluffy. Add melted chocolate and vanilla; continue beating until shiny and smooth.
Place 1 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.
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