Here are some short cuts if you run out of some ingredients!
Buttermilk - 1 cup of milk (cows, soy, almond, rice, etc) to 1 tablespoon of apple cider vinegar.
Powdered Sugar - 1 cup of white granulated sugar to 1tablespoon of corn starch - blend well in a blender until it's a fine powder
Heavy Cream - 3/4cup of milk (cows, soy, almond, rice, etc) to 1/3 cup butter. Melt the butter, then stir into milk.
Vegan Egg Substitute - 1 tablespoon of ground flax seeds to 3 tablespoons of cold water. Mix well until gelatinous