Gently pluck the yellow petals from about 4 cups of dandelions, discarding any green parts which can add bitterness. Rinse petals in cold water until no dirt remains and drain.

In a medium saucepan, bring 4 cups of water to a boil. Remove from heat, add the petals and a strip of lemon zest if using, cover, and steep for 20–30 minutes.

Line a fine mesh sieve with cheesecloth and strain the petal infusion into a clean pot, pressing gently to extract liquid without forcing bitter green bits through.

Return the liquid to the saucepan and measure exactly 3 cups of the infusion if needed. Stir in 4 cups granulated sugar until dissolved. Bring the mixture to a boil over medium-high heat.

Stir in the powdered pectin and 1/4 cup lemon juice. Stir constantly; the mixture will foam a bit and turn glossy. If foam forms, stir in the teaspoon of butter to reduce it.

Boil hard for 1 minute, stirring constantly, then remove from heat. The jelly should cling to the back of a spoon, thickening as it cools.

Ladle hot jelly into sterilized jars, leaving 1/4-inch headspace. Wipe rims clean, seal with lids, and process in a boiling water bath for 10 minutes for shelf stability or simply refrigerate for up to 3 weeks if not water-bath canning.