Preheat your oven to 350°F. Spray a 9x13-inch metal baking pan with nonstick spray and line with parchment paper, leaving overhang on two sides for easy removal.

In a large bowl, melt the 14 1/2 tablespoons of butter. Whisk in the dark brown sugar and granulated sugar until well combined. Add the eggs and vanilla, whisking vigorously for 30-45 seconds until smooth and glossy.

Stir in the flour, baking soda, baking powder, and salt until just combined. Fold in the chocolate chips. Divide the dough into two equal halves (about 640 grams each). Press one half into the prepared pan, then lift out the parchment with the dough and set aside on a baking sheet. Replace the parchment in the pan with a fresh sheet.

In a medium saucepan over medium heat, melt the cubed 3/4 cup butter. Once melted, reduce heat to medium-low and cook, stirring frequently, until the butter turns a deep golden brown and smells nutty and caramel-like, about 3-6 minutes. Remove from heat.

Immediately stir in the chopped semi-sweet chocolate until completely melted and smooth. Whisk in the vegetable oil and cocoa powder. Set aside to cool slightly (it should be warm, not piping hot).

In a large mixing bowl, whisk together the 3 eggs, egg yolk, and granulated sugar for 1-2 minutes until the sugar dissolves and the mixture is pale and slightly thickened. Whisk in the powdered sugar, vanilla, and salt.

Gradually whisk the warm chocolate mixture into the egg mixture until smooth. Fold in the flour, espresso powder (if using), and the 1 cup of chocolate chips until just combined.

Spread slightly more than half of the brownie batter evenly in the bottom of the prepared pan. Carefully place the reserved sheet of cookie dough on top, then peel off the parchment paper. Spread the remaining brownie batter over the cookie dough layer.

Divide the remaining cookie dough into large, thick discs and place them on top of the brownie batter, spacing them evenly. If desired, press a few extra chocolate chips into any bare spots on the cookie dough.

Bake for 38 to 44 minutes, until the cookie tops are golden brown and the brownie surface is shiny and set. A toothpick inserted into a brownie area should come out with a few moist crumbs, not wet batter. Do not overbake.

Let the bars cool completely in the pan on a wire rack for about 1-2 hours before slicing. Sprinkle with flaky sea salt just before serving, if desired. Store leftovers in an airtight container at room temperature for up to 4 days. Reheat individual slices in the microwave for about 15 seconds for that fresh-from-the-oven taste.