Sift flour once, measure, add baking powder and salt, and sift together three times. Cream butter thoroughly, add sugar gradually, and cream together well. Add egg yolks and mix well. Add chocolate and vanilla and blend. Add flour and mix well. Beat egg whites until stiff enough to hold up in moist peaks, but not dry. Fold into chocolate mixture. Turn into greased 9x9-inch pan. Bake in slow oven (325 degrees F.) 30 to 35 minutes. Cool. Cut into 16 pieces.
To serve, place ice cream between two chocolate chews and serve with Chocolate Sundae Sauce. Serves 8.