Without brown sugar, the simplest fix is to work a little molasses or honey into white sugar by hand until it turns moist; this mix reproduces both the colour and the mild caramel note of brown sugar fairly closely.
What makes brown sugar different?
Brown sugar is refined white sugar with molasses added back in, and that molasses is what gives it its dark colour, its moist, clumpy texture and its faint caramel taste. In cakes and cookie dough, brown sugar leaves the result noticeably moister and darker than white sugar does.
So swapping in plain white sugar keeps a bake sweet enough, but the texture comes out drier and the colour paler.
How do you fake brown sugar at home?
Stirring a tablespoon of molasses or dark honey into a cup of white sugar with a fork quickly produces a moist, clumped mixture that can be used cup for cup in place of brown sugar. Caramelised sugar is another option, though it takes more care and time to get right.
Coconut sugar is also close in colour and carries a light caramel note; it can be swapped in at a one-to-one ratio, and because its sweetness is similar it doesn't throw off a recipe's texture much.
Where does it make the biggest difference?
In cookies and brownies, where a moist crumb matters, using only white sugar instead of brown noticeably dries things out — the molasses mixture is especially worth using there. In syrup-soaked desserts the colour brown sugar would add is largely masked once the syrup itself darkens on the stove, so plain white sugar works fine there too.
Why does moisture content matter so much?
Brown sugar's moisture content directly affects a cake batter's consistency; a molasses-moistened sugar mix adds a touch more liquid than plain white sugar would, which can mean adding a pinch more flour in some recipes. If you add too much molasses or honey while mixing, the batter can turn runny — a few more spoonfuls of white sugar brings it back into balance.
If the mixture isn't used right away it can clump and harden quickly, so it's best to make only as much as the recipe calls for and use it the same day; kept in the fridge, it can stiffen within a few days and need working through again.
If you use honey instead, keep in mind it has a stronger flavour than molasses; a neutral pine honey rather than a floral one keeps the recipe's own aroma from being overpowered. Since even one extra spoonful can shift the overall balance, it's safest to add it gradually and taste as you go.
If you want to keep the mixture in a jar, sprinkling a thin layer of white sugar on top slows moisture loss and helps it stay soft for a few extra days.