Design Eat Repeat

0 Preheat oven to 350°F.
2 In a medium-sized microwave-safe bowl, melt unsalted butter ([adjustable]¾ cup, i.e. 1 ½ sticks[/adjustable]) for 30-60 seconds until fully melted. If your butter is steaming, let it cool down for a few minutes. Butter that is too hot can result in greasy bars and/or will cause the eggs to cook.
3 Add in light brown sugar ([adjustable]¾ cup) and granulated sugar (½ cup[/adjustable]) and use a spatula to stir until combined. Stir in [adjustable]1 whole egg, 1 extra egg yolk and vanilla extract (2 teaspoons)[/adjustable]. Mix until combined.
11 Stir in baking soda [adjustable](¾ teaspoon) and salt (¾ teaspoon)[/adjustable].
13 Add in unsweetened cocoa powder ([adjustable]¾ cup) and all-purpose flour (1 ½ cups[/adjustable]). Before all the flour disappears, add in [adjustable]1 ¼[/adjustable] cups of chocolate chips. Mix until all the flour disappears into the dough. Immediately proceed to next step.
4 Press dough into a greased or parchment-lined metal 9x13" pan until smooth. Sprinkle the remaining [adjustable]1/4 [/adjustable]cup chocolate chips on top and gently press each one into the dough.
6 Bake at 350°F for 14-16 minutes, or until center no longer looks wet. Do not overbake or the bars will not be soft & may taste bitter. Remove from oven and place entire pan on a cooling rack. If desired, sprinkle with flaky sea salt.
12 Allow the cookie bars to cool completely before removing from pan. Slice into 12-24 squares, then enjoy! To keep fresh, cover pan with saran wrap or place bars in airtight continer for up to 2-3 days (best within 2 days or they start to dry out). See notes below for freezing tips.