Beat butter at medium speed of an electric mixer 2 minutes or until creamy. Gradually add 1 1/2 cups sugar, beating well. Add egg and beat well.
Combine flour, cream of tartar, and salt; add to butter mixture, beating at low speed just until blended.
Roll dough to 1/4" thickness between 2 sheets of wax paper. Cut with cookie cutters. Place 1" apart on ungreased cookie sheets.
Combine egg yolk and water; stir well. Divide mixture among several cups. Tint with desired colors of food coloring. Keep egg yolk paint covered until ready to use. Add a few drops of water if paint thickens too much.
Using a small paintbrush or sea sponge, paint assorted designs on some cookies with egg yolk paint. Sprinkle some cookies with decorator sugar. (Some cookies will be baked plain.)
Bake at 350° for 10 to 12 minutes. Let cookies stand 1 minute on cookie sheets. Carefully transfer cookies to wire racks to cool completely. Frost remaining cookies with vanilla and chocolate frosting and decorate as desired.