Heat oven to 350 F. Lightly coat 9" square baking pan with nonstick cooking spray.
Mix together flour and baking powder in a small bowl. Beat together banana, brown sugar and granulated sugar in bowl until blended. Add buttermilk, egg, oil and vanilla, beating well.
Stir in flour mixture just until blended. Fold in nuts. Spread batter evenly in prepared pan.
Bake in 350 F. oven for 25 to 30 minutes or until wooden pick inserted in center comes out clean. Cool in pan on wire rack.
Meanwhile, stir together confectioner's sugar and water in small cup until smooth. If necessary for a better drizzling consistency, add more water, a drop at a time. Drizzle over cooled cake. Cut cake into 12 bars.