How to Make It
Preheat oven to 325°.
Stir together Streusel Topping ingredients (brown sugar, 1/3 cup granulated sugar, cinnamon, and chopped nuts) in a small bowl until combined. Set aside.
Cream butter and sugar at medium speed with an electric mixer until light and fluffy. Add eggs, 1 at a time, beating just until yellow disappears.
Sift together flour and next 3 ingredients; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Stir in vanilla. Pour batter into a greased 9- x 13-inch pan; sprinkle with Streusel Topping. Repeat and sprinkle remaining Streusel Topping.
Bake at 325 for 35-40 minutes or until a wooden pick inserted in center comes out clean. Let cool 20 minutes before serving.