Preheat oven to 350 degrees.
In a small saucepan melt butter. Remove from heat.
Stir in sugar, cold water, and vanilla.
Stir in flour, cocoa, and baking powder until thoroughly combined. Stir in nuts.
Spray the bottom of an 8x4x2 inch loaf pan with nonstick spray.
Pour batter into pan.
Bake for about 20 minutes or until a toothpick inserted in center comes out clean.
Cool completely in pan on wire rack.
Remove from pan. Cut into 8 bars.
Sprinkle with powdered sugar.