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 red gel food coloring and fully incorporate it.
Santa Hat Assembly
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 red buttercream frosting. Place the frosted triangles in the fridge for 10 minutes to the set frosting.
Dip each mini marshmallow in water, shake off excess and then roll around in a small bowl of sprinkles. Place the marshmallow at the top of the triangle.
Roll out the fondant to ⅛" thickness. Cut strips of the fondant to fit the bottom of the triangle, about ½" wide.
Take the strip of fondant and swipe the top with water using your finger. Dip the wet side into the sprinkles and then place on top of the frosted triangle.
Notes
To get a nice pressed rice cereal treat, spray your hands with non-stick spray before pressing them into the pan.
If you don't have fondant, you can also use white frosting or line up some mini marshmallows for the hat trim.
The thick buttercream frosting is the easiest to work with when cold.
The crystal sprinkles stick well with just water! If you don't have the sprinkles these treats still look great without them.
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.