Preheat oven to 400°F. Roll sheets of aluminum foil into 3 loose rolls, each measuring approximately 17 inches long, 1 1/2 inches wide, and 4 inches tall. Place rolls parallel on a rimmed baking sheet.

Stir together sugar and cinnamon in a small bowl. Brush melted butter evenly on both sides of tortillas, and sprinkle both sides evenly with sugar mixture. Drape 2 tortillas over each foil roll, so that they form a taco shell shape.

Bake tortillas in preheated oven until tortillas hold their shape and are lightly browned, about 15 minutes. Transfer shells to a wire rack, and cool completely.