In large bowl of an electric mixer, beat butter and sugar until creamy.
Beat yolk into butter mixture; cover and reserve white.
In another bowl, stir together flour and cinnamon; add to butter mixture, using your hands if necessary to blend thoroughly.
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 in a 275° oven for 1 hour or until firm when lightly touched.
While still hot, cut into 1 1/2-inch squares.
Let cool in pan on a rack.