Preheat oven to 350°.
Coat a 13 x 9-inch baking pan with cooking spray; dust with 2 teaspoons flour.
Lightly spoon 2 2/3 cups flour into dry measuring cups, and level with a knife. Combine with baking powder and next 4 ingredients (baking powder through cloves) in a bowl.
Beat 2 egg whites at high speed of a mixer until soft peaks form. Gradually add 1/2 cup brown sugar, 1 tablespoon at a time, beating until stiff peaks form.
Beat butter and 1 1/2 cups brown sugar at medium speed of a mixer until well-blended. Add eggs, 2 egg whites, and vanilla; beat well. Add flour mixture to sugar mixture alternately with buttermilk, beginning and ending with flour mixture. Pour batter into prepared pan. Carefully spread egg white mixture over batter. Sprinkle with walnuts. Bake at 350° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool completely on a wire rack.