Prepare a large baking sheet by lining with baking paper.
Melt the chocolate over low heat, slowly adding the milk until well incorporated. The chocolate should be runny.
Drizzle half the chocolate over popcorn and immediately add 1/3 of the sprinkles. Toss gently with a rubber spatula to get the popcorn lightly covered in chocolate. Don't over stir or your sprinkles might melt.
Add the rest of the chocolate and 1/3 of the sprinkles and toss again.
Place the mixture on the baking sheet, separating the popcorn pieces.
Add the rest of the sprinkles and transfer to the fridge.