Place the butter in a saucepan over medium-low heat, stirring until melted. Add the marshmallows and continue cooking over low heat, stirring often, until melted.
Drizzle the melted marshmallow mixture on top of the popcorn, stir to coat, and spread out evenly again. Melt the candy melts according to package directions and drizzle on top of the popcorn.
While the candy melt drizzle is still wet, scatter the heart sprinkles and Valentine's M&Ms on top. Move the baking sheet to the refrigerator for 20 minutes to chill.