Preheat the oven and prepare the pan

• Preheat the oven to 350 °F (175 °C).
• For thicker, fudgier brookies, grease and line an 8x8 inch square pan with parchment paper; for thinner, crisper brookies use a 9x9 inch pan.
• Set the pan aside.

Brown the butter for the cookie dough

• Add the butter to a saucepan over medium-high heat.
• Melt and brown it while stirring continuously until golden.
• Remove from the heat and immediately pour into a large bowl; set aside to cool.

Brown the butter and make the brownie chocolate mixture

• Brown the remaining butter in a saucepan the same way.
• While still on the heat, add the cocoa powder and quickly stir to combine.
• Remove from the heat, let stand, then mix in the dark chocolate chips and olive oil until smooth; set aside to cool.

Mix the cookie dough dry ingredients

• In a medium bowl, whisk together the flour, baking soda, baking powder and salt.
• Set aside.

Finish the cookie dough

• Once the cookie brown butter has cooled slightly, whisk in both sugars.
• Whisk in the egg and vanilla until lighter in color, then whisk in the heavy cream.
• Fold in the dry ingredients until just combined, then fold in the semi-sweet chocolate chips. Cover and chill in the fridge.

Make the brownie batter

• In a large bowl, whisk the eggs with both sugars until the sugar is dissolved and the mixture is frothy, then whisk in the vanilla.
• Stream in the cooled chocolate mixture while whisking until fully incorporated.
• Fold in the flour and salt, then fold in the chocolate chips.

Assemble the brookies

• Spread half of the brownie batter into the prepared pan.
• Form large discs of half the cookie dough and dollop them over the batter, leaving some brownie batter visible.
• Spread the remaining brownie batter on top and finish with dollops of the remaining cookie dough.

Bake the brookies

• Bake at 350 °F (175 °C) until golden brown, with set golden edges and a slightly soft middle.
• Check at 30 minutes; if the top browns too much, tent with foil.

Cool and cut

• Let the brookies cool in the pan for at least 20 minutes.
• Carefully remove from the pan and cut into 9 or 16 squares with a sharp knife.

Serve

• Sprinkle with flaky sea salt and serve with a cold glass of milk.