Heat oven to 350 degrees. Lightly spray an 8 -inch square baking pan with vegetable spray.
In medium saucepan over low heat, melt spread. Add sugar; stir until well blended. Remove from heat; stir in cocoa and vanilla.
Add egg whites; stir to blend. Stir in flour and walnuts. Spread batter into prepared pan. Bake in preheated 350-degree oven for 25 minutes or until edges begin to pull away from sides of pan. Cool in pan on wire rack. Sprinkle powdered sugar over top. Cut into squares.