Grease a 9x13-inch baking pan with non-stick cooking spray.Place your marshmallows and butter in a large microwavable mixing bowl. Microwave for 1-2 minutes or until marshmallows gets puffy.
Mix in the Fruity Pebbles and stir until combined. Press the Fruity Pebbles firmly into the prepared pan.
Freeze for 30 minutes.
Use your cookie cutter to cut out as many Christmas trees as you can.
Frost your trees by drawing garland onto your trees using a back and forth movement. Or decorate as desired.
Dab a small amount of frosting onto the back of one M&M and stick it onto the top of the tree for the star.
Serve Fruity Pebble Christmas Trees as a fun holiday treat.