Preheat the oven to 350 degrees. Grease an 8 inch square pan with butter, or, if you'd like to be able to remove the entire slab of cookies, grease and line with parchment paper, allowing it to go several inches up the sides to create a "sling."
Combine the butter, brown sugar, and white sugar in a bowl. Beat together until smooth. Add the egg, egg yolk, vanilla, syrup, baking soda, and salt, beating until combined.
Stir in the peanut butter followed by the oats. Stir in the chocolate chips.
Spread the dough into the prepared pan. Dot with M&Ms, pressing them gently into the dough.
Bake for 16-17 minutes or until the top is light browned, but not completely set. Let the bars set up and cool completely before slicing.