Harvest and prepare the petals. Pick bright, open dandelion flowers early in the morning from pesticide-free areas. Pluck the yellow petals away from the green base, rinse gently in cool water and drain.

Steep the petals. Place petals in a large saucepan with 4 cups water. Bring to a gentle simmer, then remove from heat, cover and let steep for 12 to 24 hours.

Strain the infusion. Line a sieve with cheesecloth or a jelly bag and strain the infusion into a bowl, pressing gently to extract as much liquid as possible without forcing bitter bits through. Measure the resulting liquid; add water if needed to make 3 cups.

Prepare to cook. Return the measured dandelion infusion to a large saucepan, stir in powdered pectin and mix well. Bring to a full rolling boil over medium-high heat.

Add sugar. Add the 4 cups sugar all at once, stirring constantly. Boil hard for 1 minute while stirring, then remove from heat. Stir in the lemon juice and optional butter.

Jar and process. Skim any foam from the top, ladle hot jelly into sterilized jars, leaving 1/4-inch headspace. Wipe rims, apply lids, and process in boiling water bath for 10 minutes.

Cool and wait. Let jars cool completely, then wait 24 hours to check seals before storing.