Cream butter and sugar thoroughly. Beat in egg yolk.
Add flour and cinnamon, blending well.
Roll pieces of dough between buttered palms into 1" balls.
Place balls on ungreased baking sheet about 2" apart.
Press out paper thin with a floured spatula.
Paint with egg white and sprinkle with colored candies or ground nuts.
Bake at 350F for 10-12 min.
Makes 6 dozen 2 1/2" cookies.