1️⃣ Prepare the popcorn:
Add 13-16 cups of popped popcorn to a large mixing bowl. Ensure there’s enough room to stir and coat the popcorn later.

2️⃣ Melt the butter and marshmallows:
In a large saucepan over medium-low heat, melt 1 cup of butter. Add the mini marshmallows and stir continuously until the mixture is completely smooth and melted.

3️⃣ Add brown sugar and flavoring:
Stir in ¾ cup light brown sugar until it dissolves completely (this should take about 1 minute). Remove the pan from heat and mix in ½ teaspoon of vanilla extract, if using.

4️⃣ Coat the popcorn:
Pour the warm marshmallow caramel mixture over the popcorn in the bowl. Stir gently but thoroughly to ensure the popcorn is evenly coated. If the texture is too sticky or clumpy, add a bit more popcorn until the consistency is just right.

5️⃣ Spread and garnish:
Spread the coated popcorn onto a sheet of wax paper in an even layer. While it’s still warm, sprinkle flake sea salt for a salty-sweet balance, or add mini M&M’s for a pop of color and chocolatey goodness.

6️⃣ Cool and serve:
Let the popcorn cool for 10–15 minutes to allow the caramel coating to set slightly. Serve immediately or store in an airtight container for up to 2 days.