Pop the kernels in an air popper or on the stovetop until you have 10 cups of fluffy popcorn, then transfer to a large mixing bowl.

Drizzle the hot popcorn with 3 tablespoons of melted butter and toss gently until every kernel is lightly coated.

Place a medium saucepan over low heat, add 4 cups mini marshmallows and 2 tablespoons butter, and stir constantly until completely melted and smooth, about 3–5 minutes.

Remove the pan from heat and stir in the salt, vanilla extract, and cinnamon if using.

Immediately pour the hot marshmallow mixture over the buttered popcorn and fold with a buttered spatula until the popcorn is evenly coated, about 2 minutes.

Grease your hands with butter or cooking spray. While the mixture is still warm but cool enough to handle, scoop roughly 1/2-cup portions and press gently to form 12 compact balls.

Set the balls on a parchment-lined tray and let them cool at room temperature for at least 20 minutes to firm up before serving or storing.