Preheat your oven to 350℉ (about 175℃).

Spray a 9x13 metal pan with nonstick spray and line it with parchment paper.

Melt 14.5 tbsp of butter in a large mixing bowl.

Whisk in the dark brown sugar and granulated sugar until combined (about 30-45 seconds).

Add the room temp eggs and vanilla, mixing until smooth.

Stir in the dry ingredients (flour, baking soda, baking powder, and salt), then fold in the chocolate chips.

Divide the dough into two equal halves.

Press half of the dough into your prepared pan.

In a medium saucepan, melt 3/4 cup of cubed butter over medium heat.

Stir occasionally until golden brown and fragrant (about 3-6 minutes).

Stir in the chopped chocolate until melted, then whisk in the vegetable oil and cocoa powder. Set aside to cool.

In a new bowl, whisk together the eggs, egg yolk, and granulated sugar.

Mix in the powdered sugar, vanilla, and salt.

Add the warm (not hot) chocolate mixture to the egg-sugar mixture.

Fold in the flour and espresso powder (if using), then add the chocolate chips.

Spread more than half the brownie batter in the bottom of the lined pan.

Lay the cookie dough on top, then spread the remaining brownie batter on top.

Form extra cookie dough into thick discs and press them on top.

Bake for about 38 to 44 minutes.

The top should have a shiny brownie crust.

A toothpick will come out with moist crumbs but not wet batter.

Let them chill in the pan for about 1-2 hours on a wire rack.

Sprinkle with flaky sea salt if desired.

Store leftovers in an airtight container at room temp for 3-4 days.