Preheat oven to 350°F (180°C). Line a 9″x13″ (23x33 cm) baking dish with parchment paper.
If using graham crackers (not crumbs), grind the crackers in a food processor to form crumbs.
Mix together the graham cracker crumbs and melted butter until well combined.
Press the mixture on the bottom of the pan.
Pour both cans of condensed milk over the graham cracker crust and gently spread to coat the crust completely.
Sprinkle on the chocolate chips, chopped pecans, and coconut shreds, in that order. Top with the M&M's, if using.
Bake for 25-30 minutes, until the top is golden-brown.
Let cool completely before cutting and serving. These are a bit messy so if necessary, chill in the fridge or freezer before cutting.