Line a half sized baking sheet with baking paper. Have your other ingredients ready in bowls nearby as once the chocolate is melted you want to move fairly quickly.
Create a double boiler to melt the chocolate: Bring a half full small/medium saucepan of water to the boil. Reduce to simmer. Place a heatproof bowl over the saucepan, so that it sits steady. Add the white chocolate. Stir to melt this. OPTIONAL FOR FOOD COLOR: Once melted, remove a large tablespoon or two of the chocolate into a small bowl. Add a drop of food color and stir to combine.
Pour the white chocolate over the lined baking sheet. Spread it into an even layer with a spatula. If using, add the colored chocolate on top, swirling it in with a spoon.
Sprinkle over the cereal, marshmallows and sprinkles. Let firm up in the refrigerator for 2 hours or so before breaking up with your hands. Enjoy!