Pop your popcorn according to package directions. Add it to a large baking sheet that's been lined with wax paper. Discard any unpopped kernels. 

Add your butter, vanilla extract, and marshmallows to a medium bowl and microwave in 30-second increments in the microwave (stir in between). I err on the side of microwaving less and letting it sit a bit longer between intervals so I don't scorch it. 

Once everything is melted together nicely, give it a good stir and then pour it over the popcorn. Toss it until it's fully coated. I use two spoons to do this.

Spread the popcorn out in an even layer and add the sprinkles. 

Let the popcorn set. I like to speed this process up by putting the baking sheet in the fridge for 15 minutes or so. 

Keep popcorn in a sealed container or portion the popcorn up (e.g. in small bags). It'll last for a few days without going stale. This popcorn mix is quite sticky, so you could even make popcorn balls out of it if you wish.