Beat butter at medium speed with an electric mixer until fluffy; gradually add sugar, beating until blended. Add egg, sour cream, and vanilla; beat well.
Combine flour and next 5 ingredients; gradually add to butter mixture, beating until blended. Pour batter into a greased, wax paper-lined 9-inch pan.
Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes. Remove from pan; let cool on a wire rack 10 minutes. Pour Macadamia-Fudge Topping over cake. Chill until ready to serve. Garnish, if desired.