On a sheet pan, broil the jalapeños until they are nicely charred and blistered. Then turn the oven to 425°. Add the brown sugar to the jalapeños and roast for 15–18 minutes. Let peppers cool and add them (and the caramelized brown sugar in the pan) to the tequila in a jar and let meld until ready to use (at least a couple days). The longer you infuse the tequila, the spicier it will be.