Line a 9x13 inch baking pan with parchment paper.
Add butter to a large pot, and melt over low heat.
Add the marshmallows, and heat, stirring often, until melted.
Add the food coloring, and stir until the color is solid.
Stir the Rice Krispie cereal into the melted marshmallows.
Pour mixture into the prepared baking dish, and press to the sides.
Refrigerate for 1 hour.
Remove from the refrigerator and lift the entire parchment sheet out of the baking dish, and onto a large cutting board.
Using a large knife, make a diagonal cut, then another the opposite way, making two trees. Continue cutting out tree (triangle shapes), until you cannot cut anymore. You could also use a Christmas Tree cookie cutter for this step.
Set the Christmas tree cutouts onto a parchment lined baking sheet.
Pour the candy melts into a piping bag (or ziplock bag), and heat in the microwave for 30 second intervals, until melted. Massage bag with your hand between each heating session.
Snip the end off the piping bag (or ziplock baggie), and add a small amount to the bottom of each Mini Reece’s Peanut Butter Cups, then stick one to the bottom of each tree.
Drizzle melted green candy in a back and forth motion over the top of each tree.
Add sprinkles on top of the melted candy.
Store in an airtight container in a cool location.