Line two large baking sheets with parchment paper or wax paper.

In a very large bowl, gently toss together the popped popcorn and the square cereal.

In a microwave-safe bowl, melt the white chocolate chips (and coconut oil, if using) in 30-second increments at 50% power, stirring well after each increment until completely smooth.

Pour the melted white chocolate over the popcorn and cereal mixture. Gently fold with a spatula until evenly coated.

While still sticky, gently fold in the pastel marshmallows and candy-coated chocolates.

Spread the mixture in a single, even layer onto the prepared baking sheets.

Immediately sprinkle the rainbow sprinkles evenly over the top of the wet mixture.

Let the mix sit at room temperature for 30-45 minutes (or 15 mins in the fridge) until the chocolate hardens. Break into pieces and serve.