Preheat oven to 375°. Coat a 9-in. square baking pan with cooking spray., Cream butter, sugar substitute and brown sugar until light and fluffy. Add vanilla and eggs, one at a time, beating well after each addition. Stir in water. Whisk together flour, milk powder, cocoa, baking powder, baking soda and salt; add to creamed mixture just until combined. Stir in bananas., Transfer to prepared pan. Bake until a toothpick inserted in center comes out clean and cake begins to pull from sides of pan, 23-28 minutes. Cool completely in pan on a wire rack. Dust with confectioners' sugar.