Line the bottom and sides of a 9x13 baking pan with parchment, leaving about an inch overhanging the top; set aside until ready to use. Melt butter in a large pan1 over medium heat. Add marshmallows and stir with a silicone spatula until completely melted. Remove the pan from the heat and immediately stir in the Fruity Pebbles.
Then, quickly transfer the mixture to the prepared pan. Use the spatula to press the mixture into an even layer, making sure to compact it down. Allow to cool for 30 minutes.
Line a tall microwave-proof glass with a piping bag. Place Ghirardelli melting wafers into the piping bag and heat in the microwave according to package directions. Lift the parchment out of the pan and peel it away from the sides of the Fruity Pebbles Treats, laying it as flat as possible. Snip the tip off the piping bag and drizzle the white candy melts over the top in a zig-zag pattern. Allow the candy to harden before cutting into squares. Store in an airtight container.