In a large microwave-safe bowl, melt the butter. Add the melted butter to the bowl of marshmallows and toss to coat. Microwave for 2–3 minutes, or until the marshmallows puff up and begin to melt.

Stir the mixture, then mix in the vanilla extract.

Add the Rice Krispies cereal and stir until everything is evenly coated.

Press the mixture into an 8 x 11.5 inch baking dish lined with parchment paper. Use a wet spatula to spread it out evenly.

Let sit at room temperature for about 1 hour, or until firm.

Lift the treats out of the pan using the parchment paper.

Using a knife, cut them into 2 sections. Then cut each section into large triangles (about four per side) to create tree shapes.

Divide the icing into two bowls. Place about 1/3 of the icing in one bowl and color it yellow. Color the remaining icing green. Transfer each color into separate piping bags fitted with a star piping attachment (optional).

Pipe green icing onto each triangle, starting just below the top and swooping down side to side to mimic branches. Add a yellow dot of icing at the top of each tree for the star. Top with sprinkles.