1. Preheat oven to 350*F. Line bottoms of two 9in round cake pans with waxed paper. Grease paper and sides of pans. Dust with flour.
2. Mix flour, cocoa, baking soda, and salt. In another bowl, beat 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.
3. At low speed, alternately beat flour mixture and buttermilk into butter mixture just until blended. Divide batter equally between prepared pans.
4. Bake cakes until a toothpick inserted in center comes out clean, 25-30 minutes. Transfer pans to wire racks to cool for 10 minutes. Turn out onto racks. Remove paper. Turn layers top side up and cool completely.
5. To prepare frosting, beat butter and confectioner's sugar at medium speed until tight and fluffy. Add melted chocolate and vanilla; continue beating until shiny and smooth.
6. Let assembled cake set for 30 minutes before garnishing with chocolate shavings.
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