Prepare a large baking sheet by lining it with parchment paper. Set aside.
In a microwave-safe bowl, microwave chocolate bark, half the vanilla bark, and 1 1/2 Tbsp butter in 30-second intervals until completely melted. Stir after each cooking interval until smooth.
Allow chocolate mixture to cool a bit before adding marshmallows and walnuts. Stir until marshmallows and walnuts are completely coated.
Using a cookie scoop or tablespoon, scoop clusters onto your prepared baking sheet. Allow clusters to cool and harder.
Melt remaining vanilla bark and 1/2 tbsp butter in a small microwave-safe bowl for 30 seconds. Using a spoon, drizzle over clusters. Add sprinkles before vanilla bark hardens. Let cool until set.