Prepare your milkshake glasses first: Use a knife to edge the tops of your glasses with about 1 inch of frosting. Press M&M'S into the frosting to create a trail of candies around the tops of the glasses.
Prepare the milkshakes: Place ice cream, ½ cup milk, cake mix and M&M'S in a blender. Blend until smooth, adding a little more milk if too thick.
Fill milkshake glasses and garnish with fancy straws. Add mini cupcakes punched through the straws, if desired (just use sharp scissors to snip a hole into the bottoms of the cupcake liners).
Notes
You can certainly make a bunch of mini milkshakes for a party, if you'd like. Just use smaller glasses or cups and follow the same instructions.