Are they interchangeable?
Raise your hand if you’ve been in the midst of scooping brown sugar into a batter only to realize the recipe you’re using calls for dark brown sugar. Huh? You look again at your container to see that it says “light brown sugar.” Wait, there are differents degrees to which sugar can be brown? Yep. Are they really two different products? Absolutely. But this doesn’t mean you’re SOL.
Most brown sugar you’ll find at the store is actually refined white sugar that has been mixed with molasses. You can even make your own in a pinch. Cook’s Illustrated says that light brown sugar can be recreated by mixing 1 tablespoon molasses into 1 cup granulated sugar (that’s a 1:16 ratio), and dark brown sugar can be replicated by mixing 2 tablespoons of molasses into 1 cup of granulated sugar (a 1:8 ratio). You can pulse the mixture together in a food processor or simply mix the ingredients into your recipe.
Still, you’re wondering if this means that now you need to keep two types of brown sugar on hand. Not really. For the most part, dark and light brown sugars can be used interchangeably in recipes, as technically dark brown sugar simply has a slighter higher molasses content. However, more molasses does mean dark brown sugar has slightly higher levels of acidity and moisture. This might mean that, for example, a batch of cookies baked with dark brown sugar instead of light may rise a bit higher and taste a bit more caramel-y than those made with light brown sugar. Still, the difference is ultimately nominal. Pick one type of brown sugar to keep in your pantry and bake freely.
Perhaps you’ve heard of another molasses-y sugar: muscovado. Also known as muscovado brown sugar, it’s actually the “real deal” brown sugar, an unrefined cane sugar that leaves the molasses that naturally occurs in place. With a deep, roasted flavor, muscovado can also be used interchangeably with standard store-bought dark or light brown sugar. While muscovado is more expensive than the other brown sugars, it will lend a more complex flavor to your baked goods.