1/2 cup butter
1/2 cup brown sugar, firmly packed
1 tablespoon grated lemon rind 
1 teaspoon vanilla extract
2 eggs, separated
1 cup sifted flour
1/4 cup water
6 ounces semisweet chocolate chips
2 tablespoons confectioners sugar

Preheat oven to 375 degrees F.

Blend butter, 1/4 cup of the brown sugar, lemon rind and vanilla extract. Beat in egg yolk. Add flour alternately with water. Add semisweet chocolate chips.

Beat egg white until stiff but not dry. Add remaining 1/4 cup brown sugar gradually until the mixture is stiff and satiny. Fold into semisweet mixture. Spread in wax paper-lined 12 x 8-inch pan. Bake for 25 to 30 minutes. Cool, then sprinkle with confectioners' sugar. Cut into squares.