"A ruby red, fruity marmalade. I've never tried any commercial prickly pear jam or jelly, so I don't know if this is how it's supposed to taste, but my toddler does keep asking for more, so it's a win for our family. Store any extra jam in the refrigerator."
Bring a large pot of water to a boil. Add six 4-ounce jars; simmer to sterilize. Wash lids and rings in warm soapy water.
Slice ends off each prickly pear. Make 1 long vertical slit down each one. Use the slit to hold the skin and peel off. Discard peel. Cut flesh into quarters and transfer to a saucepan; mash with a potato masher.
Simmer prickly pear flesh over medium-low heat until soft, 30 to 45 minutes. Press cooked flesh through the fine screen on a food mill set over a bowl.
Measure 1 1/4 cup prickly pear pulp into the saucepan. Add sugar, lemon juice, lemon zest, and butter; bring to a boil. Stir in pectin until dissolved, about 1 minute.
Pour prickly pear mixture into hot jars, filling to within 1/4 inch of the top. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids and screw on rings.
Place a rack in the bottom of a stockpot and fill halfway with water. Bring to a boil and lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water if necessary to bring the water level to at least 1 inch above the tops of the jars. Bring the water to a rolling boil, cover the stockpot, and process for 5 minutes.
Remove the jars from the pot and place onto a cloth-covered or wood surface, several inches apart, and let cool, at least 24 hours.