Put milk chocolate chips into a medium-sized bowl and microwave in 20-second increments, stirring after each, until smooth, about 1 minute total.
Pour brownie mix into chocolate, 1 tablespoon at a time, stirring until smooth after each addition.
Pour chocolate onto a baking sheet lined with parchment paper or a silicone liner. Spread smooth to desired thickness.
Microwave white chocolate chips in a medium-sized bowl in 20-second increments, stirring after each, until smooth, about 1 minute total.
Scoop white chocolate by spoonfuls onto milk chocolate and, using a knife or a toothpick, swirl around to create a marbled effect. Top with sprinkles if desired.
allow to harden at room temperature until chocolate is completely cooled and hard.