Cream butter, sugar, vanilla, and salt until light and fluffy. Mix in egg, beating well. Add cake flour gradually, blending well (dough should be stiff). Lightly grease a 10/15 jelly roll pan (cookie sheet). Spread dough in pan, being sure to spread evenly to all sides and corners. Bake at 250 for 20-25 minutes. Spread chocolate ion top. When cake is finished, put in oven for a few minutes. Cut into squares before completely cooled. Makes about 3 - 3 1/2 dozen squares.