Cut chocolate into 1-inch pieces and place in heat-proof bowl. Set over low heat in small skillet containing 1/2-inch simmering water. Stir occasionally just until chocolate is melted. Remove from heat and set aside.

In mixer bowl combine all ingredients except flour and walnuts; beat to blend thoroughly. Mix in flour. Spread batter in prepared pan; sprinkle with walnuts.

Bake about 30 minutes until springy to the touch about 2-inches around edges. Cool on rack. Cut into 11/2-inch squares.