In a small saucepan melt margarine; remove from heat. Stir in sugar, water, and vanilla. Stir in flour, cocoa powder, and baking powder until well mixed. Stir in chopped nuts.
Spray the bottom only of an 8x4x2-inch loaf pan with nonstick cooking spray. Pour batter into pan.
Bake in a 350 degree F oven about 20 minutes or until a toothpick inserted near the center comes out clean. Cool thoroughly. Remove from pan. Cut into 8 bars. Sprinkle with the powdered sugar. Makes 8 servings.