Ingredients

• Cooking spray

• 1 1/2 c. 

  (320 g.) packed light brown sugar

• 1 c. 

  (2 sticks) unsalted butter, melted

• 2 

  large eggs

• 2 tsp. 

  pure vanilla extract

• 1/2 tsp. 

  baking soda

• 1/2 tsp. 

  kosher salt

• 2 c. 

  (240 g.) all-purpose flour

• 1 c. 

  (170 g.) semisweet chocolate chips

• 1 c. 

  (180 g.) M&M’s

Description

1. 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.
2. 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.
3. Step 3Using a flexible spatula, fold in flour, chips, and M&M’s until just combined.
4. Step 4Transfer dough to prepared dish and smooth in an even layer.
5. 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. 
6. Step 6Make Ahead: Bars can be made 1 week ahead. Store in an airtight container at room temperature.