Cream butter and sugar together until light; beat in egg yolk and vanilla.
Sift flour and cinnamon together; stir into creamed mixture.
Beat egg white lightly.
Place dough in ungreased 15 1/2 x 10 1/2 x 1-inch baking pan; brush with egg white.
Sprinkle with walnuts, pressing into surface lightly.
Bake at 350 degrees for about 20 minutes.
Cut into diamond shape while warm