Sift together flour, cocoa, baking soda, and salt in a small mixing bowl.
In a separate bowl, cream 3/4 cup butter and white and brown sugars at medium speed until light and fluffy. Add eggs, one at a time, beating after each addition. Add 2 teaspoons vanilla. At low speed, begin by adding a little of the flour mixture to the butter mixture. Alternate with buttermilk until all are blended together. Start and end with flour.
Line bottoms of 2 9-inch pans with waxed paper; grease and flour. Pour batter in.
Bake in a preheated 350 degrees F (175 degrees C) oven for 25-30 minutes. Test with toothpick. Cool in pans for 10 minutes before turning onto racks to cool completely.
To Make Frosting: Cream together 3/4 cup butter and confectioners’ sugar. Slowly add melted chocolate and 2 teaspoons vanilla. Beat until light and fluffy. Frost cooled cake.
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