Bake the cupcakes according to the directions on the box. Bake time may vary. Let cool before decorating.
Make the frosting by adding the room temperature melted butter to a large mixing bowl. Mix on medium speed for 1 minute using a stand mixer or hand mixer. Add the vanilla extract, confectioners' sugar, and 1 tablespoon of milk. Mix on low speed until combined. If needed, add an additional tablespoon of milk at a time and mix again until you reach the desired consistency.
Fill the piping bag with the vanilla buttercream frosting. Pipe onto the cupcake in a single layer, forming a large swirl.
Using the black decorating icing, draw the eyes toward the top of the marshmallow. Then, draw little dots in a curved pattern to create the mouth. Using the orange decorating icing, draw a line between the eyes and the mouth to create the nose.
Attach the marshmallow to the cupcake by gently inserting a pretzel into the marshmallow and sticking it into the frosting to create the snowman’s head.
Stick two pretzels on both sides of the head into the cupcake to create the arms. Press 3 M&M's into the frosting to create the snowman’s buttons. Enjoy!
Notes
If you're making these ahead of time, consider frosting and decorating them the day of the event to keep the marshmallows and pretzels fresh.