Add the sugar and molasses into the bowl of a stand mixer. Use 3 tbsp molasses for dark brown sugar. If mixing by hand, place ingredients into a spacious, sturdy mixing bowl.
2 tablespoons unsulfured molasses, 1 cup granulated cane sugar
Mix on low speed to combine, then raise to medium speed until blended thoroughly. If mixing by hand, massage the molasses into the sugar using a spoon. Using the back of the spoon, press any clumps into the side of the bowl until the mixture is uniform.
Store it in a sealed, airtight container to keep it moist.
Notes
To soften brown sugar if it hardens:
Add a slice of bread or apple to the container, close it tightly and allow it to sit overnight or for a few days if the sugar is rock hard.
You can also add the hardened sugar to a microwave-safe container with a few drops of water, cover the container and microwave on High for 20 seconds.
You may also warm it in an oven-safe dish with a few drops of water at 200°F/100°C for 15 minutes or until softened.