In a mixing bowl, cream butter and sugar until light; add eggs and beat well. Sift together the flour, soda, salt, and nutmeg; stir into the creamed mixture. Stir in the buttermilk,, mashed banana, and vanilla. Spread batter in an 11x7-inch baking pan.
Combine topping ingredients and sprinkle over top. Bake at 350° for 40 to 50 minutes, or until a wooden pick or cake tester inserted in center comes out clean.
Calories per serving: 302
Fat per serving: 11.3g