Preheat oven to 350 and spray mini muffin pan with baking spray.
In a medium sized mixing bowl, combine sugar cookie mix, softened butter, and egg; mix until no clumps remain. Stir in sprinkles.
Scoop the prepared dough into each mini muffin hole, filling about 3/4 of the way full. Use the back of your spoon to create an indention and then use your hands to press the dough against the sides of the cup.
Bake for 9-11 minutes, until edges are slightly golden. You may need to press your spoon into each cookie cup once more to get the “cup” shape.
Allow to cool completely in the pan, 20-25 minutes.
To prepare the frosting, add softened butter and powdered sugar to a mixing bowl and mix until creamy. Add vanilla, milk, salt, and green food coloring. Mix together until smooth and creamy.
Pipe or spoon buttercream frosting into each cookie cup and top with 3 m&m’s.