Preheat oven to 350º. Grease and lightly four two 9” round cake pans.
Mix flour, cocoa, baking soda, and salt in a medium bowl, set aside.
In a medium bowl, beat butter, sugar and brown sugar with an electric mixer 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 until just blended. Divide batter evenly into prepared pans. Bake cakes for 25-30 min. or until a toothpick inserted in center comes out clean. Transfer pans to wire rack and cool for 10 min. Turn out onto racks and cool completely.
Beat butter and confectioners sugar at medium speed with an electric mixer until light and fluffy. Add melted chocolate and vanilla; continue beating until shiny and smooth. Place 1 cake layer, top side down on platter, spread with 1/3 of frosting. Top with other layer, top side up, and frost top and sides with remaining frosting. Let stand 20 min. before serving.