Using raw sugar in a recipe that calls for superfine sugar can leave you with a mousse or souffle that's lumpy, unevenly cooked and less than photo-ready. However, superfine sugar isn't as easy to find in stores as plain granulated sugar, so you might be forced to use a substitute.
Granulated sugar is the best substitute for superfine sugar because it's essentially the same product; the only difference is that the crystals of granulated sugar are slightly larger. If you have a blender or food processor, you can even make superfine sugar by using granulated sugar. Simply measure out the amount of sugar that your recipe calls for, tip it into the blender or processor and pulse it several times or blend on low speed until the crystals are smaller and evenly ground. Otherwise, OChef.com notes that it's fine to substitute an equal amount of granulated sugar for superfine in any recipe that involves baking.
Cookies, brownies and other baked goods are likely to turn out softer and less crispy if you use powdered sugar as a substitute for superfine sugar, but it will work in a pinch. Powdered sugar crystals are so small that the sweetener is powdery and soft like flour and doesn't seem to have a crystallized structure at all. Substitute an equal amount of powdered sugar for superfine sugar, or use a blend of powdered sugar and granulated sugar to get a crisper final product.
Brown sugar has crystals that are about the same size as those of granulated sugar, but the sweetener is slightly softer, stickier and damper because it contains molasses. It's great for providing a crisp texture and robust flavor and works particularly well as a superfine sugar substitute in recipes with rich, spicy flavor notes. In those, substitute an equal amount of brown sugar for superfine sugar. However, brown sugar is not as successful in pound cake, sugar cookie or plain cake recipes because it will add a tinge of molasses flavor to any baked good.
You can also use honey, maple syrup, agave or other liquid sweeteners to substitute for superfine sugar. Each liquid sweetener has a distinct taste that will translate to the final product, so be aware of that when baking. Liquid sweeteners are also less likely to produce a crisp or crunchy cookie than superfine, granulated or brown sugar. Most importantly, they tend to be sweeter on the tongue than superfine sugar, so if you're substituting with them, use only 3/4 of the sugar amount in the recipe, as AllRecipes.com recommends.