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.