Melt your butter and marshmallows in a large bowl in the microwave or over medium low heat in a large saucepan. This will take roughly 2-3 minutes.Stir together until marshmallows are completely melted.
Pour in your Rice Krispies cereal, Fruity Pebbles and white chocolate chips. Stir together until cereal is completely coated.
Line a 9x13inch pan with parchment paper. Pour cereal into a and press down evenly. (Don't squish too much!)
Let chocolate set completely and cut. Store at room temperature in an airtight container for 7 days.