Prep a 9x13 pan by spraying with non-stick spray or greasing with butter.
Melt butter and marshmallows in a large, deep pan. (I find my ceramic-coated dutch oven is the perfect amount of nonstick!)
Add Fruity Pebbles and stir to combine.
Pour the cereal mixture into the prepared pan. Spray a bit of non-stick spray on the bottom of a glass or on your hands to press the mixture into an even layer.
Let the cereal bars set up for about 20 minutes.
If using the optional topping: In the meantime, melt the white chocolate, coconut oil, and extract (if using) in a double boiler OR using the microwave in 30-second bursts. Stir until smooth, well combined, and a drizzling consistency.
Cut the set cereal bars into rectangles like granola bars, or squares if you prefer. If you're using the topping, set the bars on top of a piece of parchment with a bit of space between.
If using the optional topping: Dip a fork in the melted chocolate mix and Jackson Pollock spatter over the top of the sliced cereal bars. Sprinkle the crushed strawberries on top.
Let the white chocolate set (you can speed this up by popping the tray into the fridge for about 15 minutes). Dig in!