Beat butter, dark brown sugar, and granulated sugar at medium speed with an electric mixer until combined. Add eggs, 1 at a time, beating until blended after each addition. Add vanilla, beating until blended. Add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Pour batter into a lightly greased 9-inch square pan; sprinkle evenly with Streusel Topping.
Bake at 350° for 35 to 40 minutes or until a wooden pick inserted in center comes out clean. Serve warm or at room temperature.