This is from Ingrid Hoffman, "Simply Delicioso," on Food Network. It has only 4 ingredients and can be made in less than an hour. You can be creative using different colored M&Ms for holidays or special occasions.
Butter a 9x9 inch square baking dish. Line with plastic wrap, allowing it to overlap on the ends. (The butter will keep the cake from sticking to the sides, and the plastic wrap will make the cake easy to lift out of the pan.).
Melt butter in a saucepan.
Add marshmallows and melt over low heat, stirring occasionally with a wooden spoon.
Remove from heat. Stir in popcorn very quickly and mix well.
Fold in M&Ms using a spatula, again working quickly to prevent the candy from melting.
Pour into pan, pressing the mixture firmly into the pan with the back of a wooden spoon.
Lift plastic wrap out of the pan and invert onto a plate. Peel the plastic wrap off the cake.