Add the brown sugar and water to a saucepan or medium heat. Then let it come to a simmer (not a boil).
Use a whisk to combine the sugar into the water and turn the heat off once the sugar is dissolved. Let the brown sugar syrup cool down to room temperature before storing and refrigerating. Use it in iced coffees, brown sugar boba, or as a syrup for soaking cakes.
Notes
Soften your brown sugar- If your brown sugar is in a hard clump because it's been exposed to air for too long, soften it by microwaving it covered by a damp paper towel for 20 seconds.
Don't boil it too long- You don't want to overheat this brown sugar simple syrup or it will get very thick and sticky.
Let it cool- This step allows the syrup to cool and thicken. Wait until it's at room temperature before refrigerating it.
Thicken it- If you want it to be thicker, simmer it for a minute more, then let it get cool. Refrigerating it will also increase the viscosity.