Beat butter and sugar at medium speed with an electric mixer until creamy. Add egg, beating well. Stir in squash and vanilla.
Combine flour and next 4 ingredients; gradually add to butter mixture, beating at low speed after each addition. Pour batter into a greased 9-inch square pan. Sprinkle with Streusel Topping.
Bake at 350° for 40 minutes. Cool cake in pan on a wire rack.