Husarenkrapfen-Thumb Cookies (Germany)

1 1/4 sticks butter or margarine
1/2 cup sugar
1 egg, separated
1 2/3 cups flour
Grated lemon rind
Coarse sugar
Jelly/jam or nuts

Cream butter and sugar thoroughly; add egg yolk and mix well. Blend in flour and lemon peel. Mix thoroughly. Form dough in long roll, the size of a 50 cent piece in diameter. Cut in ¾ inch thick slices; make thumb print in center of each and dip top side of cookie in unbeaten egg white, then in coarse sugar. Put a dot of jelly/jam or a nut in center. Line up on ungreased baking sheets and bake at 350 degrees until bottom of cookie is medium brown. Yield 2-3 dozen