Step 1: Prepare your canning jars by washing them in hot, soapy water and rinsing thoroughly. Keep them hot (e.g., in a simmering water bath or a low oven) until ready to fill. Set aside lids and bands.
Step 2: While wearing gloves, carefully slice the jalapeño peppers into uniform 1/8 to 1/4-inch thick rounds. You can use a sharp knife or a mandoline for consistency. Remove seeds and membranes for a milder candy, or leave them for more heat.
Step 3: In a large, non-reactive pot (stainless steel is ideal), combine the apple cider vinegar, granulated sugar, turmeric, celery seed, garlic powder, and cayenne pepper. Stir well to mix all ingredients.
Step 4: Bring the mixture to a rolling boil over medium-high heat, stirring frequently until the sugar is completely dissolved. This will create your sweet and spicy brine.
Step 5: Add the sliced jalapeños to the boiling brine. Reduce the heat to a gentle simmer and cook for 5 to 7 minutes, stirring occasionally, until the jalapeños slightly soften and turn a darker shade of green.
Step 6: Using a slotted spoon, carefully transfer the softened jalapeño slices into the hot, sterilized canning jars, leaving about 1/2 inch of headspace at the top of each jar. Pack them fairly tightly.
Step 7: Return the remaining brine in the pot to a high boil. Let it boil vigorously for 5 to 7 minutes, allowing it to thicken slightly into a syrupy consistency. It should coat the back of a spoon.
Step 8: Carefully pour the hot, thickened syrup over the jalapeños in the jars, ensuring all the peppers are submerged and maintaining 1/2 inch of headspace. Use a non-metallic utensil to remove any air bubbles.
Step 9: Wipe the rims of the jars clean with a damp cloth. Center a hot lid on each jar, then screw on the bands until they are fingertip-tight (do not overtighten).
Step 10: If canning for shelf stability, process the jars in a boiling water bath canner for 10 minutes. If not canning, allow them to cool completely, then refrigerate. For canned jars, carefully remove them from the canner and let them cool undisturbed on a towel-lined counter for 12-24 hours. Check seals (lids should be concave).
Step 11: Store sealed jars in a cool, dark place for at least 2 weeks before opening to allow flavors to meld. Once opened, store in the refrigerator and consume within a few weeks.