Beat butter at medium speed with an electric mixer until creamy; add sugar, beating until fluffy. Add egg, beating well.
Combine flour and next 3 ingredients; add to butter mixture alternately with milk, beginning and ending with flour mixture. Stir in morsels and vanilla. Pour into a greased and floured 8-inch square pan.
Bake at 350° for 20 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack 10 minutes. Invert onto wire rack; cool. Spread with Mint Chocolate Frosting.