If a recipe specifically calls for light or dark brown sugar, but you don’t have the right one, you can actually make your own from other ingredients in the pantry. Brown sugar is just like white sugar, but with the addition of molasses. So, all it really takes to make your own brown sugar is white sugar, molasses, and some multiplication skills.
To make light brown sugar, take the amount of sugar the recipe calls for and multiply it by 0.04. This will tell you the amount of molasses needed, and then you mix that with white sugar. This method is easiest when thinking in terms of weight. For example, if a recipe calls for 1 cup of light brown sugar you would need to think of 1 cup as 16 tablespoons, or 48 teaspoons. Perform the math and you end up with about 2 teaspoons of molasses.
To make dark brown sugar, use the same method as above, but multiply the amount of sugar by 0.07 and add that amount of molasses.
If you don’t have molasses, it’s okay to simply use white sugar in place of brown, but you should add 1/4 teaspoon of cream of tartar or white vinegar for every cup of white sugar in the recipe.
Read more about brown sugar substitutions here.