How to Soften Brown Sugar In a Microwave. You can use the microwave as a quick way to soften the brown sugar. Just follow these tips: Place the brown sugar in a microwavable bowl. Put a couple of damp paper towels on top of the brown sugar. Tightly cover the bowl. Heat the bowl in the microwave for 30 seconds. If not yet softened, continue to heat for 10- second intervals. Fluff the sugar with a fork and use immediately.