Preheat oven to 275F. In large bowl with an electric mixer, beat butter and sugar until creamy. Separate egg, then beat yolk into butter mixture; cover and reserve the egg white. In another bowl, stir together flour and cinnamon; add to butter mixture. With your hands, spread dough evenly over bottom of a greased 10 by 15-inch rimmed baking pan. Beat egg white lightly, then brush over dough to cover evenly. Sprinkle pecans over top; press in lightly. Bake for 1 hour or until firm when lightly touched. While still hot, cut into 1/2-inch squares. Let cool in pan on a rack. Store airtight.