Spray a 9x9 inch square pan with non-stick cooking spray or line with parchment paper.
Place the butter, marshmallows, and a few drops of green food dye in a large non-stick pot.
Carefully melt on low heat, stirring frequently until melted.
Add the Rice Krispies to a bowl and stir with the melted marshmallows and butter until well combined.
Spoon the mixture into the prepared pan, spread evenly. Spray your clean hands with non-stick cooking spray and press mixture down.
Set pan aside for about 20 minutes to cool and set.
Remove the Rice Krispies block from pan and transfer to a cutting board.
Using a Christmas tree cookie cutter, cut out the Christmas Tree shapes.
In a small microwave safe bowl, Heat the white melting chocolate for 30 seconds, then stir until smooth. Continue heating for 30-second intervals, as needed until smooth.
Transfer melted chocolate to a Ziploc bag and clip the corner. Drizzle over the Christmas trees and sprinkle with Christmas sprinkles.