Design Eat Repeat

0 Preheat oven to 350°F.
2 In a large bowl, use an electric mixer to cream unsalted butter [adjustable](¾ cup, i.e. 1 ½ sticks), granulated sugar (¾ cup), and light brown sugar (½ cup)[/adjustable] for 2 minutes until light in color.
3 Add in eggs [adjustable](2 large), light corn syrup (3 tablespoons), and vanilla extract (2 teaspoons)[/adjustable]. Mix until combined.
16 Add peanut butter [adjustable](¾ cup) and mix until combined. Stir in baking soda (1 teaspoon) and salt (½ teaspoon)[/adjustable]. 
13 Add all-purpose flour [adjustable](1 ¾ cups) and quick oats (1 ¾ cups)[/adjustable] and mix until just combined.
14 Mix in chocolate chips [adjustable](¾ cups) and m&m's (¾ cups)[/adjustable]. Note: For this step, note that we're only using [adjustable]¾ [/adjustable]cups of chocolate chips & [adjustable]¾[/adjustable] cups m&m's inside the dough so that we can reserve the other ¼ to sprinkle on top.
4 Drop dough into a greased or parchment-lined metal 9x13" pan and spread until smooth. The dough will be a little sticky, so it's easiest to drop the dough into small pieces instead of adding it in one big blob. Sprinkle the remaining ¼ cup chocolate chips and [adjustable]¼ [/adjustable]cup of m&m's on top and gently press into the dough.
6 Bake at 350°F for 28-32 minutes until edges and top are lightly golden brown and the center no longer looks wet. Do not overbake or your bars can get dry and will not be as soft and chewy. You want the cookie bars to be lightly brown on the edges and top. Remove from oven and place entire pan on a cooling rack. 
12 Allow the cookie bars to cool completely before removing from pan. If you cut the bars when they're still warm, they will be crumbly and fall apart. For the cleanest cuts, they need to be fully cooled and/or slightly chilled before cutting.
15 Slice into 12-24 squares, then enjoy! To keep fresh, cover pan with saran wrap or place bars in airtight container for up to 3 days (after that they start to dry out). See notes below for freezing tips.