Preheat oven to 350 degrees. Prepare mini muffin pan by spraying each cup liberally with non-stick cooking spray. Pour chocolate cake mix into a medium bowl and whisk {or sift} to remove any lumps. Add eggs and oil to the cake mix and combine with a hand held mixer {or by hand} until dough is thick and sticky. Using a small cookie scoop {or 1/2 tbsp.}, put a mound of cookie dough into each muffin cup. Using your fingers, go back and press the mound of dough down to create a level cookie. Bake for 9-11 minutes. {Do not over cook, they will set up as they cool.} Remove from oven and immediately run a knife around the edge of each cookie cup {I used an offset butter knife}. Go back and use a tart shaper {or end of a wooden spoon} to press middle of cookie down and create an indentation. Move cookies to a cooling rack and let them cool completely. Continue to bake second batch if necessary.
Assembling Pinata Cookie Cups
Once the cookie cups are completely cooled, fill each cookie cavity {that you created with the tart shaper} with about a tablespoon of mini M&M's. Fill a piping bag or Ziploc bag with chocolate frosting and pipe over the top of the M&M's covering the candy, starting on the outside and working in {be sure to cover all of the M&M's}. Once all of the pinata cookies have frosting, add a shaped sugar topper or extra Mini M&M's. Store in an air tight container at room temperature for up to 5 days.