Preheat the oven to 350° F. Line sheet trays with parchment paper, and set them aside.
In a medium-sized bowl, stir together the flour, cornstarch, baking soda, and salt. Then, set the bowl aside.
In the body of a stand mixer with the paddle attachment, cream the butter with the granulated and brown sugar for 3 minutes or until it’s light and fluffy.
Add the eggs one at a time until they’re fully mixed, and stir in the vanilla.
Add the flour mixture to the bowl, and slowly stir it in until combined. Scrape down the sides as needed.
Add the M&Ms and chocolate chips to the mixture, and stir to combine. Add the sprinkles, and stir again.
Take 2 tablespoons of the dough at a time, and roll it into a ball. Form the dough so that the balls are a little bit taller than they are wide, in a spherical shape to avoid too much spreading as they bake. Continue with all the dough.
Bake for 11-12 minutes or until the cookies are lightly golden brown along the edge and the tops appear dry. Do not overbake!
When your cookies come out of the oven, press more M&M’s, chocolate chips, and sprinkles into the dough for a pretty presentation. Let the cookies cool for 10 minutes on the sheet tray. Then, transfer them to a wire rack to cool completely.
While the cookies are baking, make the filling.
Cream the butter in the body of a stand mixer with the paddle attachment until smooth.
Add the powdered sugar a little at a time until it is fully mixed in. Add the salt, vanilla, and milk. Stir to combine. Then, place the mixer on medium-high speed for 3 minutes or until the mixture is fluffy.
Divide the frosting evenly between 3 bowls. Add red food coloring to one bowl and green food coloring to another bowl. Stir each bowl to combine until you reach your desired color of frosting. Leave the last bowl as is.
Place the frosting in three piping bags or three plastic bags, and snip off the end. Pipe a dollop of each color onto the bottom of half of the cookies. Place another cookie on top, and gently press down to form a sandwich. Serve immediately, and enjoy!