You can go up or down, so to speak.

That is, you can use 1/3 cup of regular white sugar, and it will probably be delicious. You could use 1/3 cup of dark brown sugar, with good results – but we don't think you'd bother writing us if you have dark brown sugar on hand.

The best choice (why didn't we say it first?) would be to mix 1/3 cup of white sugar with 1 tablespoon of molasses. If you do have dark brown sugar on hand, you can also mix 1/6 cup (5 tablespoons + 1 teaspoon) of white sugar with 1/6 cup dark brown sugar, and, now that we think of it, that would be the easiest and least fussy choice (really, you should be reading this from the bottom up).

Finally, if you have something like turbinado sugar or raw sugar on hand, you can substitute that for the light brown sugar.