Start by popping your popcorn using an air popper or stovetop method until fluffy, aiming for approximately 8 cups of popped corn. Sift through it for unpopped kernels, then place in a large mixing bowl to cool slightly.

In a medium saucepan, melt the butter over medium heat. Once melted, add the brown sugar, stirring constantly for 2-3 minutes until smooth and bubbling gently.

Remove from heat and stir in the marshmallows and vanilla extract, stirring for another 2-3 minutes until melted and smooth.

Immediately pour the warm caramel over the popcorn, folding it gently with a spatula to ensure every piece is coated before the caramel sets.

Spread the caramel-coated popcorn onto a parchment-lined baking sheet and let it cool for 10-15 minutes until set.

Break the popcorn into clusters and serve in bowls or bags. Optionally sprinkle with flaky sea salt before serving.