Preheat oven to 350 degrees. Line a rimmed baking sheet with parchment paper.
In a large bowl, mix together cake mix, eggs and vegetable oil until combined. Stir in M&Ms. Drop rounded tablespoons of dough onto the prepared cookie sheets.
Bake in preheated oven for 8-10 minutes. Remove from oven and let cool.
For frosting: Beat butter until smooth. Working in batches, mix in powdered sugar. Add milk 1 Tbsp at a time until frosting is desired consistency. Spread on cooled cookies and top with additional M&Ms or sprinkles if desired.