Preheat over to 350F. Melt butter in a large saucepan, add sugar, flour, egg yolk and vanilla. Mix well. Spread evenly in a 9x12 inch baking pan. Spread lightly beaten egg whites over top of mixture and sprinkle evenly with chopped nuts. Bake 20 minutes or until lightly browned. Remove from oven and let sit in pan for 3 to 4 minutes. Cut into squares while still in pan and allow to cool thoroughly before removing.
Makes about 2 dozen.
Cajun Country -- New Iberia, Louisiana