I found this hidden in a recipe that I StumbledUpon:

If needed, you can make your own brown sugar by adding 1/2 – 1 tsp of molasses to 1/2 cup of sugar and mixing well. It takes a little while to mix completely, but you can make it as light or as dark as you wish. It’s my understanding that commercial brown sugar is made by mixing molasses back in to processed sugar, so it’s really not any different than store-bought.

Brown Sugar and Peach Muffins (spelt challenge!)