In a bowl, combine sugar and egg white. Blend with your fingers.
Pack sugar mixture firmly into front and back halves of a plastic skull mold. Scrape tops level with a spatula. Return extra sugar mixture to bowl.
With a spoon, gently scoop sugar mixture from the center of each mold to make a shell 1/2 to 5/8 inch thick.
Invert a baking sheet over each mold; holding mold and pan together, invert again. Tap mold gently and lift carefully off sugar skull. If skull breaks, wash and dry mold, then fill again, reusing sugar mixture. (If mixture sticks to mold, dump back into bowl and stir in 2 to 4 tablespoons sugar; if mixture crumbles, return to bowl and mix in more egg white, 1 teaspoon at a time.) Repeat, using remaining sugar mixture, to make a second skull if desired.
Bake in a 200° regular or convection oven until skull surface feels hard and solid when lightly pressed, 20 to 30 minutes. Let cool.
To join front and back halves of each skull, spread rims with icing (recipe follows) and fit pieces together; rub off any icing that oozes from seam. Let stand until icing is firm, at least 1 hour.
Using a pastry bag with plain or decorative tips, pipe icing onto skulls to decorate as desired. Let dry at least 1 hour. To store, wrap airtight.
Icing: In a bowl, with a mixer on high speed, whip 1 large egg white and 1/8 teaspoon cream of tartar until foamy. Gradually beat in 1 1/2 cups powdered sugar. For intense colors, blend in food coloring paste as desired. For delicate shades, use liquid food coloring. If necessary, beat in more powdered sugar to make icing stiff enough to hold its shape when squeezed through a pastry bag. Makes about 3/4 cup, enough to join and decorate one or two skulls.