Brownie batter cookie dough:

1. Add the chocolate and butter to a microwave safe bowl, and microwave in 15 second increments, stirring in between, until the chocolate has melted
2. Add the cocoa powder to the chocolate and butter, and stir until combined. Set aside to cool down
3. Separately, add the egg, egg yolk, brown sugar, granualted sugar and vanilla to a bowl. Whisk on high speed for 3-4 minutes until the mixture is light in color and almost triple in volume
4. Pour the chocolate mixture into the egg mixture, along with the coffee dissolved in water, and whisk on low speed until just combined
5. Sift in the flour, baking powder, baking soda and salt. Use a rubber spatula to gently fold in until the flour is almost mixed. Add the chopped milk chocolate and continue gently folding until the ingredients are just combined
6. The batter will have a consistency in between batter and cookie dough - don't worry, it will firm up and become scoopable while you make the chocolate chip cookie dough

Chocolate chip cookie dough:

1. Whisk the butter, granulated sugar and light brown sugar in a large bowl at medium high speed for 2-3 minutes until the mixture is fluffy and light in color, scraping the bowl along the way
2. Add the egg, egg yolk, vanilla, and instant coffee dissolved in water. Whisk for another minute to combine
3. Sift the flour, baking powder, baking soda and salt in a separate bowl and mix to combine
4. Add the dry ingredients to the cookie dough in 2 batches, and fold gently with a rubber spatula. When almost all the flour has mixed in, add the chopped chocolate and continue mixing until combined. Do not overmix

Assembly and baking:

1. By the time the cookie dough is done, the brownie dough should be firmer and scoopable. If it isn't, put it in the fridge for 5 minutes. Similarly, if the cookie dough is too sticky, refrigerate for 5-10 minutes until it is a scoopable consistency
2. Lay out parchment paper on 2 baking sheets. Use a round tablespoon measure to scoop the brownie batter cookie dough into ~1oz balls on one sheet (you can use a weighing scale to be precise)
3. Now scoop the chocolate chip cookie dough into 1 oz balls on the other sheet
4. Hold one hand open and place a chocolate chip cookie dough ball on it. Then place a brownie ball right next to it. Keep repeating and going in a circle until you're holding 6 balls alternating between each flavor (see pictures in blog post above). Gently press the balls together and roll them into 1 giant ball that has a marbled look. This ball will be ~6oz
5. Repeat until you have 6 brookie cookie dough balls. This recipe results in a few extra brownie dough, but you can just bake small brownie cookies with it separately! Optionally, top the cookie dough balls with some more chopped chocolate.
6. Refrigerate the cookie dough balls overnight (or a minimum of 6 hours)
7. When ready to bake, pre-heat your oven to 375F (conventional, no gas) and line a large cookie sheet with parchment paper
8. Place the cookie balls onto the cookie sheet, keeping at least 3" between them (use 2 sheets if necessary).
9. Put the cookies in the oven. Bake for 5 minutes at 375F, then drop the temperature to 340F and continue baking for another 11-13 minutes for a total baking time of 16-18 minutes. When perfectly baked, the cookies should be golden in color, feel set around the edges, and soft to touch in the middle 
10. Remove the cookies from the oven, sprinkle with some flakey sea salt and add some more chocolate on top of them. Let them rest for at least 20 minutes before serving!