In a mixing bowl with a wooden spoon, cream together the sugar, butter, and cream cheese . Gently incorporate the flour. Divide the dough into two flat rounds, each about 1 inch thick, and wrap in wax paper. Refrigerate at least 6 hours or overnight.

Preheat the oven to 350°F. On a lightly floured surface, roll out the dough very, very thin and cut with cookie cutters, transferring the cookies to ungreased cookie sheets. Gather up the dough scraps, knead them together, roll out, and cut again.

In a small bowl, stir together the egg yolk and cream. Brush this mixture over the top surfaces of the cookies. Then, lightly decorate the cookies with sprinkles or other decorations.

Bake the cookies just until their edges start to turn golden, checking frequently, 5 to 10 minutes depending on your oven and the thickness of the cookies. Remove from the oven promptly and, with a spatula, transfer to cooling racks. When completely cooled, store in airtight tins.