Cookie dough batter:

• Place the butter in a non-stick pan and melt it on medium-low heat. Keep stirring for a few minutes until the butter starts to look brown and develops a caramel-y nutty aroma
• Turn the heat all the way to the lowest setting, and add the brown and white sugars. Whisk on the lowest flame for 1-2 minutes, then remove from the heat. The mixture may look somewhat grainy, don't worry
• Remove from heat and cool the mixture for 5 minutes. It should be warm, but not hot anymore
• Add the egg and yolk, and quickly whisk to incorporate. Add the vanilla and instant coffee dissolved in hot water, and whisk for 1-2 minutes until the mixture becomes very smooth and creamy like caramel
• Sift all the dry ingredients into a separate bowl. Add the chopped chocolate and mix everything together
• Make sure the wet ingredients aren't hot (they shouldn't be). Add the dry ingredients to the wet in 2 batches and fold with a rubber spatula until just combined. Do not overmix! 

Brownie batter:

• Pre-heat your oven to 350F (conventional, no fan) and prepare a 9x9 square pan with parchment paper 
• Melt and brown the butter in a saucepan just like you did for the cookie dough batter
• Place the chopped dark chocolate in a heat-safe bowl and pour the hot brown butter on top. Let rest for 2-3 minutes, then stir until the chocolate melts and a smooth mixture forms
• Add the instant coffee dissolve in hot water, and sift in the cocoa powder. Whisk them into the butter and chocolate mixture until smooth
• In a separate bowl, add the eggs and sugar , and whisk for a few minutes until the mixture becomes really light in color (almost white) and thick and fluffy in texture
• Pour the chocolate mixture into the egg mixture, and whisk until combined
• Sift in the flour and salt, and gently fold until just combined
• Lastly, fold in the chopped chocolate

Assembly and baking:

• Spread a thin layer of the brownie batter in the bottom of the prepare 9x9 pan (about ~1/3 of the batter)
• Dollop on chunks of the cookie dough batter in a haphazard way, with some brownie batter still showing through (you can flatten it with your hands a bit if needed). Then spoon on dollops of the brownie batter, with some cookie dough batter still showing through. Repeat and alternate until all the cookie dough batter and brownie batter are used up. The idea is to have alternating brownie and cookie batter in every bite, so you're trying to layer and alternate them without mixing them too much (see pictures in blog post above)
• Flatten the batters lightly with a spatula. Optionally, sprinkle some more chopped chocolate on top
• Bake in the pre-heated oven for ~35 minutes 
• Let the brookies cool down in the pan (outside the oven) for at least 20 minutes. Then carefully remove and cut with a sharp knife. Serve & enjoy!

Video Tutorial: