Cream butter or margarine and shortening; gradually add sugar, beating until light and fluffy. Add egg and vanilla; beat well.
Combine flour and salt; stir into creamed mixture. Divide dough in half; cover and chill at least 1 hour.
Roll one portion of dough to 1/8 inch thickness on a lightly floured cookie sheet. Cut with assorted cutters; remove excess dough. Paint assorted designs on cookies with Egg Yolk Paint, using a small paintbrush. Bake at 375 degrees F (190 degrees C) for 9 to 10 minutes. Remove to wire racks to cool. Repeat procedure with remaining dough.
To Make Egg Yolk Paint: Combine egg yolk and water; mix well. Divide mixture into several custard cups; tint as desired with paste food coloring. Cover until ready to use. If paint thickens, add a few drops of water and mix well. Yield: 1 1/2 tablespoons.
