Preheat the oven to 350 degrees. Line a baking sheet with parchment paper or a silicone mat. Using a mixer on low speed, combine the cake mix, softened butter, and eggs together in a bowl until the dough is smooth.
Fold in the white chocolate chips and M&Ms. You can chill the dough if you like, but your cookies will still be delicious if you don't.
To make large cookies, use an ice cream scoop. Put six cookies onto a baking sheet so they have room to breathe and spread.
Bake the cookies for 18-20 minutes or until the edges are firm. They'll puff up in the oven and then flatten some once they've cooled.