Melt the butter in a medium saucepan over low heat.
Add the marshmallows and stir with a silicone spatula until completely melted.
Remove the pan from the heat and pour in the rice cereal. Fold the Rice Krispies into the marshmallow mixture, stirring from the bottom up until every piece is well coated.
Lightly grease your baking dish and spatula. Pour the cereal mixture into the prepared baking dish and press it down firmly and evenly with the spatula. Aim for about 3/4-inch thickness so the stars hold their shape when cut.
While the mixture is still slightly warm, scatter the sprinkles evenly across the top. Gently press them into place with your fingertips to help them stick to the surface.
Let the cereal slab cool and set at room temperature for 30 minutes before cutting. Once done, press a star-shaped cookie cutter straight down through the slab. You can lightly grease the cookie cutter with cooking spray to prevent sticking.
Melt the candy melts separately according to the package instructions. For the white chocolate, microwave the pieces in a microwave-safe bowl for 20 seconds at a time, stirring between intervals, until completely melted.
Transfer the candy melts and white chocolate to individual sandwich bags. Snip a very small corner, then drizzle them over each Rice Krispies star in back-and-forth lines. If using all three colors, let each color set before adding the next so they don't blend together.
Let the drizzles set at room temperature for 20 minutes or refrigerate for 5 minutes, and they're ready to serve or store!