Made-from-scratch brown sugar isn't a new idea by any stretch, but in the past, it wasn't something I ever cared about. In part, I was skeptical that a "homemade" mix of refined white sugar and mass-produced molasses could somehow taste any better than "real" brown sugar...a mix of refined white sugar and mass-produced molasses.
My logic was that, unless you bothered to start with a jar of particularly delicious small-batch molasses, there were no real gains to be had—a logic that fell apart the moment a friend of mine called in a panic because she was out of brown sugar for her favorite dessert, with guests all but on her doorstep. Guiding her through the process of making the swap was easy, she didn't have to drop everything to drive to the store, her bourbon pecan layer cake turned out beautiful, and I learned the real value of homemade brown sugar: expedience.
If you ever run out of brown sugar, falling back on a jar of molasses to whip up an emergency batch is an excellent plan B—so long as you go about it the right way. First and foremost, that means using true molasses and not blackstrap, which should never be used in any recipe unless it's called for by name (full report on that topic here). The second half of the equation is actually understanding the equation. See, store-bought brown sugar is about 10% molasses by weight, but that doesn't mean you can just multiply a given weight of white sugar by 0.1 to determine how much molasses is needed. At first blush, that calculation makes perfect sense (and that's the formula you'll see bandied about most often online), but it winds up making the brown sugar too light. Even lighter than light brown sugar, which is what most recipes have in mind.*
Unless a recipe calls specifically for "dark brown sugar," it's safe to assume light brown sugar is what the author intended. Making dark brown sugar is about adding not just more molasses but also darker molasses, and, for the purpose of this post, that's a bit more complicated a topic than I'm prepared to unpack.
For a cup of white sugar (seven ounces), 10% is about one tablespoon (3/4 ounce), which is pretty convenient for folks measuring by volume. But once the white sugar and molasses are mixed together, you've got 7 3/4 ounces total, making that molasses just 9% of the total weight. Now, you may be thinking, "Close enough," but in the realm of baking, that can be a dangerous thing. Without a full 10% molasses content, homemade versions will behave more like white sugar than brown, which can have a big impact on cookies in particular, changing how they spread, rise, and brown. You can check out a full breakdown of those differences here.
In order to accurately blend your own light brown sugar at home, it's important to remember that for however much molasses you add, that much white sugar needs to come out. Going back to my previous example, seven ounces of sugar should be reduced to 6 1/4 ounces, so that 3/4 ounce molasses will be 10% of the total weight. Spelling it out sounds complicated, but don't overthink it. All you need to remember is that 6 1/4 ounces of white sugar plus 3/4 ounce molasses equals seven ounces of brown sugar.
You can make up a big batch that will keep for about a month, but if you have a scale, you can calculate exactly how much white sugar and molasses you'll need to replace any given quantity of brown sugar. Just multiply the weight of the brown sugar by 10% to find out how much molasses you'll need, then subtract that number from the whole to determine the weight of the white sugar you'll need. If you happen to have any lightly toasted sugar on hand, perhaps left over from blind-baking a pie, it can be used like white sugar to add some caramel notes that will give homemade brown sugar a more nuanced sweetness.
The exact color of the results will depend on your specific brand of molasses and whether or not the sugar is toasted, but you can typically expect it to have a somewhat redder undertone than traditional brown sugar (likely due to the deeper brown shades found in more concentrated forms of molasses). Ounce for ounce, homemade brown sugar is more voluminous than what you find at the store, simply because it hasn't had a chance to compact over time. But, so long as it's measured accurately, in both preparation and use, it will behave in much the same way.
On the left, we have samples from a batch of my favorite One-Bowl Oatmeal Cookies made with commercial brown sugar, and, on the right, cookies made with DIY brown sugar. While they spread a little more irregularly overall, the latter cookies still averaged about three inches across. If I suddenly discovered I was 86'd on brown sugar, I'd gladly take that discrepancy over the alternative of having no cookies at all.
I'll likely always prefer the convenience of commercial brown sugar, which is both affordable, reliable, and tasty. But when push comes to shove, knowing how to make a comparable brown sugar from scratch is a trick all bakers should keep up their sleeves.