Dark Brown Sugar (Best Option)
1:1 by volumeDark brown sugar contains a higher molasses content than light brown sugar, which contributes moisture and a deeper caramel flavor essential for the mousse's complexity. The molasses also helps retain the mousse's creamy texture by interacting with the fat and air incorporated during whipping.
When substituting, measure dark brown sugar by volume equal to the brown sugar called for. Slightly pack it to mimic the original's density but avoid overpacking to prevent excess moisture. Watch for minor flavor shifts toward a richer caramel note.
The final mousse will have a flavor and texture nearly indistinguishable from the original recipe, preserving the intended sweetness and mouthfeel.