Spray/grease 11x16 jelly roll pan or large square tin.
Melt chocolate, milk and sugar over low heat. Remove from heat. Add sugar stirring well to dissolve. Cool.
Stir egg yolks into chocolate mixture. Add vanilla and chopped walnuts.
Beat egg whites until stiff but not dry. Fold egg whites and alternately the flour into the chocolate mixture only until combined.
Pour into the prepared pan. Bake about 25 minutes in 350 degree oven. Test with a toothpick. If it does not come out clean, bake a few more minutes. Cut into squares when cool.