Yes, dark brown sugar can be used as a substitute for light brown sugar. They are interchangeable in most recipes, although dark brown sugar has a higher molasses content, which gives a richer, more intense flavor and slightly more moisture. This difference will not significantly affect the outcome of most baked goods, but the taste and sometimes texture may be a bit richer and moister with dark brown sugar. If a recipe specifically calls for light brown sugar and the delicate flavor or lighter color is important, you can mix dark brown sugar with some white granulated sugar to mimic light brown sugar (about 1/3 dark brown sugar with 2/3 white sugar). However, using dark brown sugar alone is generally fine and won't ruin the recipe.
