Beat together the butter and onecup of the sugar until creamy. Beat in the egg yolks, vanilla and milk.
Sift together the flour, baking powder and salt and blend into the creamed mixture. Wrap in wax paper and chill for two to three hours.
Preheat oven to moderate (375°F.).
Roll out the dough on a lightly floured cloth or board until thin; cut into shapes with cookie cutters. Arrange the cookies on lightly greased baking sheets. Brush with the unbeaten egg whites. Sprinkle the remaining sugar on the cookies and top with the citron. Bake for about eight minutes, or until lightly browned. Cool on a rack.