Preheat oven to 350°F Coat a 9-inch-square baking pan with cooking spray.
For the topping, combine flour, brown sugar, cinnamon and butter in a food processor and process until mixture resembles fine crumbs. Stir in walnuts.
For the cake, combine flour, baking soda, baking powder, nutmeg and salt in a medium bowl.
Beat butter with a mixer at medium speed about 30 seconds or until smooth. Gradually add sugar and beat 3 to 4 minutes or until fluffy. Add eggs 1 at a time, beating well after each. Add vanilla and bananas and beat until well blended. Add flour mixture alternately with milk, mixing after each addition only until smooth. Pour batter into prepared pan; sprinkle with topping.
Bake about 35 minutes or until toothpick inserted in center comes out clean. Cool in pan on a wire rack. Serve warm or at room temperature.