Step 2
In a medium bowl, whisk the melted butter, brown sugar, and granulated sugar together until no sugar lumps remain. Whisk in the egg, then the egg egg yolk, followed by the vanilla. Pour the wet ingredients into the dry ingredients and mix together with a large spoon or rubber spatula. Fold in the chocolate chips and M&Ms. Cover the dough and chill for at least 2 hours (or up to 3 days).