Blend jalapeños, optional seeds, garlic, water, and vinegar until smooth (2 min).
Transfer to saucepan; whisk in sugar and salt.
Boil, then simmer until reduced to 2 cups (about 45 min), skimming foam.
Let cool 5 min, then blend again briefly until smooth.
Cool completely, transfer to a jar or squeeze bottle, and refrigerate at least 1 day. Keeps up to 3 months.