Tip: The membranes that hold the seeds are the spiciest part of chile peppers (that's where the capsaicin is). The seeds pick up some spiciness by association. You can customize the heat of salsa or hot sauce by using some or all of the seeds along with the flesh of the pepper and tasting as you go. Be sure to wash your hands thoroughly after chopping hot peppers or wear rubber gloves.
To make ahead: Cover and refrigerate for up to 1 month or freeze in an airtight container for up to 6 months.