Preheat oven to 350°F. Grease a 9x13x2-inch baking pan.
Sift together flour, baking powder, baking soda and salt.
Cream together butter and sugar in mixing bowl until light and fluffy, using electric mixer at medium speed. Add eggs, one at a time, beating well
after each addition. Blend in bananas and vanilla. Add dry ingredients alternately with milk to creamed mixture, beating well after each addition.
Stir in nuts if using.
Pour batter into prepared pan and bake for 35 minutes or until cake tests done. Cool in pan on rack. Dust with confectioners' sugar before serving, if desired.
Makes about 16 squares.