Prepare a 9/13 pan with non-stick spray. Melt the butter in a large pot over medium heat.
Add the marshmallows and stir until fully melted in the butter.
Remove from the heat and stir in the rice cereal. Add to the prepared dish and press into the pan as much as you can. Allow the treats to cool in the pan.
Thick Buttercream Frosting
Place the butter and shortening in a stand mixer or mixing bowl and cream together on medium speed. Add the vanilla and salt and continue mixing.
Begin to add confectioners sugar one cup at a time, mixing on low speed until combined then med-high speed for a minute or so each time. After each cup addition, add a tablespoon of water to get your desired consistency. Add the gel food coloring and fully incorporate it.
Turn the pan with the pressed rice cereal treats over onto a cutting board. Cut off the edges with a sharp knife and then cut into equal triangles.
Frost each triangle with a thick layer of buttercream frosting. Place the frosted triangles in the fridge for 10 minutes to the set frosting.
Cut a pretzel in half to make the antlers and place them on top of the triangle (see post for detailed instructions).
Add the candy eyes and nose.
Buttercream frosting can be made ahead of time and stored in an airtight container in the fridge for up to 3 days before using.
If you have leftover frosting, you can freeze it in a sealed container for up to 3 months.