In a medium saucepan, bring sugar, vinegar, ½ cup (120 grams) water, mustard seeds, garlic powder, and salt to a boil over medium heat, stirring occasionally until sugar dissolves. Add jalapeños; cook, stirring occasionally, until jalapeños are darkened and just soft, about 5 minutes. Remove jalapeños using a slotted spoon, and place in a heatproof bowl.

Cook liquid over medium heat until thickened and syrupy, about 10 minutes. Pour onto jalapeños. Let cool completely. Refrigerate in an airtight container for up to 1 week.