Prepare a 9x9 baking dish with parchment paper and spray with non-stick baking spray.
In a large bowl, gently mix in the melted white chocolate into the popped popcorn. Sprinkle the sprinkles on top and dust with a little salt. Mix everything together until the chocolate and sprinkles are evenly combined.
Melt the butter in a large saucepan over medium high heat. Add the marshmallows and continuously stir to combine until the marshmallows are melted and smooth.
Stir in the popcorn and and mix until the popcorn is well coated. Immediately transfer the mixture into the prepped baking dish pressing it firmly into an even layer. Sprinkle with extra sprinkles on top if desired.
Let it harden for about 20 minutes and then cut and serve. (best when served immediately)