Lightly grease a 9×13-inch baking pan or line it with parchment paper. Set it aside before preparing the marshmallow mixture.

1. Melt the butter.Place the butter in a large saucepan and melt it over low heat.
2. Add the marshmallows.Stir in the mini marshmallows until they melt completely and become smooth. Keep the heat low to help the treats stay soft and chewy.
3. Add the flavor.Remove the saucepan from the heat. Stir in the vanilla extract, salt, and optional food coloring until evenly combined.
4. Mix in the cereal.Add the crispy rice cereal and crushed freeze-dried blueberries. Gently fold everything together until the cereal is evenly coated.
5. Transfer to the pan.Spoon the mixture into the prepared baking pan. Using a lightly buttered spatula, gently press it into an even layer without packing it too tightly.
6. Prepare the drizzle.If desired, melt the white chocolate chips with the coconut oil in short microwave intervals, stirring between each one until smooth.
7. Cool the treats.Let the mixture rest at room temperature for about 30–45 minutes, or until firm enough to slice.
8. Decorate and serve.Drizzle the melted white chocolate over the cooled treats. Allow the chocolate to set, then cut into squares and enjoy.