Pop the popcorn and measure 10 cups into a large mixing bowl. Pick out and discard any unpopped kernels so they don’t end up in the balls. Line a baking sheet with parchment paper or a silicone mat for the finished popcorn balls.

In a large pot or Dutch oven, melt the 1/4 cup (4 tablespoons) butter over low to medium-low heat. Keep the heat gentle so the butter melts but does not brown.

Add the mini marshmallows to the pot and cook over low heat, stirring constantly with a rubber spatula or wooden spoon until completely melted and smooth, 3–5 minutes. There should be no lumps.

Remove the pot from the heat. Stir in the vanilla extract and fine sea salt until evenly combined. Carefully taste a tiny bit (it will be hot) and adjust the salt if you prefer a more sweet-salty balance.

Immediately pour the melted marshmallow mixture over the bowl of popcorn. Use a spatula to gently toss and fold the popcorn until it is evenly coated, working from the bottom of the bowl up and lifting rather than mashing.

If using heat-sensitive mix-ins like chocolate chips, let the marshmallow-coated popcorn cool for 2–3 minutes so it is warm but not hot. Sprinkle in up to 1 to 1 1/2 cups of your chosen mix-ins and gently fold them in until evenly distributed without crushing them.

Let the coated popcorn sit for about 5 minutes. It should still be warm and sticky but not so hot that it is difficult or unsafe to handle. This makes shaping easier and helps the balls hold their shape.

Lightly grease your hands with butter, oil, or cooking spray. Scoop about 1/2 to 3/4 cup of the mixture into your hands and gently press and roll it into a firm ball about golf-ball to small-baseball size. Place each finished ball on the prepared baking sheet and repeat until all the mixture is used.

Let the popcorn balls cool at room temperature for at least 20–30 minutes until firm and set. For faster setting, place the baking sheet in the refrigerator for 10–15 minutes.

Once the popcorn balls are set, drizzle with melted chocolate, add extra sprinkles, or insert a paper straw or popsicle stick into each ball to make popcorn pops, if desired.