Perfect Royal Icing Recipe

Name: Perfect Royal Icing Recipe
Ingredients: 18 ounces organic powdered sugar (4 1/2 cups; 510g), divided, plus more if needed (see note); 2 1/2 ounces egg whites (1/4 cup; 70g), from 2 large eggs; 2 teaspoons (10ml) silver rum or water; 1 1/2 teaspoons (7ml) vanilla extract (see note); 1/2 teaspoon (2g) Diamond Crystal kosher salt; for table salt, use half as much by volume or use the same weight; 1/4 teaspoon cream of tartar; 1 1/2 ounces heavy cream (3 tablespoons; 40g), plus more if needed (optional; for thinning royal icing, if desired); Gel paste food coloring, optional