Before you start, sterilize your half-pint canning jars, lids, and rings according to manufacturer's directions. Keep them hot in a canner filled with hot water or a warm oven until ready to fill.

Peel, pit, and finely dice your ripe peaches. For the jalapeños, put on some gloves, slice them open, scrape out all the seeds and membranes, then finely mince them. This will give you the flavor without too much heat. You can leave some seeds in if you like a spicier kick!

In a large, heavy-bottomed pot (like a Dutch oven), combine the diced peaches, minced jalapeños, freshly squeezed lemon juice, and fine sea salt. Stir everything together gently.

Bring the mixture to a boil over medium-high heat, stirring occasionally. Once it's boiling, stir in the granulated sugar until it's completely dissolved. Continue stirring and bring the mixture to a full, rolling boil that cannot be stirred down.

Add the unsalted butter to help reduce foaming. Boil hard for 5-7 minutes, stirring constantly to prevent scorching. For best results, use a candy thermometer to monitor the temperature; the jam is done when it reaches 220°F (104°C).

Remove the pot from the heat. Stir in the liquid fruit pectin vigorously. Return the pot to a hard boil for exactly 1 minute, stirring constantly. Skim off any foam that rises to the surface.

Carefully ladle the hot jam into your hot, sterilized half-pint jars, leaving 1/4 inch headspace from the rim. Wipe the rims clean with a damp cloth, then apply the lids and screw on the rings until fingertip tight.

Process the jars in a boiling water bath canner for 10 minutes (adjust time for altitude if necessary). Once processing is complete, turn off the heat, remove the canner lid, and let the jars sit in the hot water for 5 minutes before carefully removing them.

Place the hot jars on a wire rack to cool completely for 24 hours. You'll hear the satisfying 'pop' as the lids seal. After 24 hours, check that all lids have sealed (the center should be concave and not flex when pressed). Store sealed jars in a cool, dark place for up to 1 year. Unsealed jars should be refrigerated and consumed within 3 weeks.