PREP: Preheat oven to 325°F. Spray a 9x13-inch pan with cooking spray, line with parchment (with an overhang for removal). Melt butter in microwave and set aside to cool to room temp. (If it is too hot it will melt the sugars & cause greasy bars.)
WETINGREDIENTS: Whisk melted butter and sugars until smooth. Add egg, yolk, vanilla, and syrup. Whisk until smooth and emulsified.
DRYINGREDIENTS: Use a wooden spoon to stir in baking soda, salt, and flour until just combined. Fold in chocolate chips plus M&Ms. If needed, switch to electric hand mixers, but don't over mix. Press dough evenly into pan. If desired, press a few more chocolate chips & M&Ms into tops of bars.
BAKE: 23-28 minutes until top isn't glossy, edges are lightly golden. Be careful to not over-bake for soft & chewy bars. Optional: top with sea salt right out of the oven. Fully cool in pan, lift out, cut, and serve. Best within 1-2 days.
Notes
Note 1: You can discard egg white or save and add it to breakfast the next morning (we love this Egg Skillet) or use it in our favorite Egg Wrap.
Note 2: Maple syrup is the "secret ingredient" in these bars. In a pinch, you can leave it out, but the bars do lack a bit of tenderness and a certain level of sweetness. I use pure maple syrup, not to be confused with pancake syrup.