Gaining the confidence to make substitutions is a landmark moment for any baker, because even the smallest changes can make a large difference. For example, simply swapping brown sugar for some or all of the white sugar in your recipe will give your baked goods a deeper flavor, with aromatic butterscotch overtones. Because they're essentially the same ingredient, it's an easy substitution to make.
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A Straight-Up Swap
One cup of brown sugar is a direct replacement for 1 cup of white sugar, but there's a catch. Granulated white sugar flows freely, and fills a cup densely without any help from you. Brown sugar is moist and slightly sticky, so when mounded loosely, it holds its shape instead of falling. To get the equivalent quantity of brown sugar, you'll need to compact it firmly into your measuring cup. That's why most recipes specify that brown sugar should be firmly packed. If you use a recipe with measurements by weight, you won't need to pack the sugar.
A Few Differences
Aside from the difference in flavor, brown sugar affects your baked goods in other ways. White sugar tends to make cookies crisp, while brown sugar -- thanks to its higher moisture content -- makes them chewy. By adjusting the proportion of the two sweeteners, you can fine-tune many cookie recipes to exactly the texture you prefer. The molasses in brown sugar is also slightly acidic, and will react with baking soda in your recipe to help with leavening. Dark brown sugar contains more molasses than light brown sugar, so the resulting flavor will be a bit deeper.