Dark Brown Sugar (Best Option)
1:1 by volumeDark brown sugar contains a higher molasses content than light brown sugar, which provides the moisture and rich caramel flavor critical for doughnuts. The molasses also contributes to Maillard browning during frying, enhancing crust color and taste.
When substituting, use equal volume to maintain dough hydration. Be mindful that dark brown sugar may slightly deepen the flavor and color, which is generally desirable in doughnuts.
The final doughnuts will have a moist crumb and a rich, slightly more intense molasses flavor, closely resembling those made with standard brown sugar.