Beat butter and sugars at medium speed with an electric mixer until fluffy. Add eggs, 1 at a time, beating until blended after each addition. Add cooled chocolate and vanilla, beating until blended.
Combine flour and next 3 ingredients; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed after each addition. Pour batter into 2 greased parchment paper-lined 9" round cakepans.
Bake at 350° for 22 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans, and cool completely on wire racks.
Spread White Chocolate Frosting between layers and on top and sides of cake.
Let me offer a warning about the baking instructions this recipe called for! Two 9" cake pans are not big enough! When I made this, the batter overflowed into the oven and burned pretty badly. It also took well over 22 minutes to bake.
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