Step 1Preheat oven to 350°. Thoroughly coat a 13"-by-9" baking dish with cooking spray. Fold a 16" sheet of parchment lengthwise to fit along bottom and sides of baking dish, leaving some overhang at the handles. Spray with cooking spray.
Step 2In a large bowl, whisk brown sugar and butter until sugar is dissolved. Whisk in eggs one at a time until incorporated, then whisk in vanilla, baking soda, and salt.
Step 3Using a flexible spatula, fold in flour, chips, and M&M’s until just combined.
Step 4Transfer dough to prepared dish and smooth in an even layer.
Step 5Bake bar until a tester inserted into the center comes out clean, 35 to 40 minutes. Let cool in baking dish 1 hour. Using parchment overhang, lift bar onto a cutting board. Slice into 12 bars.
Step 6Make Ahead: Bars can be made 1 week ahead. Store in an airtight container at room temperature.