1. Beat the eggs with the sugar, vanilla, and salt until fluffy. Melt the butter with the dark chocolate and stir lightly. Set aside to cool slightly.
2. Add the chocolate and butter mixture to the mixer and stir on low speed until well combined.
3. Finally, add the flour and cocoa, which you have already sifted and combined. Set aside for a while until you prepare the cookie dough.
For the Cookie Dough:
1. Melt the butter just until it's liquid, but don't let it get hot. Add the white and brown sugar along with the vanilla and whisk until the ingredients are well combined.
2. Add the egg and mix well. Sift the flour and baking powder together and stir. Add the chopped milk chocolate and stir.
3. Add the flour mixture to the mixture with the rest of the ingredients and mix very well (here you might need to mix a little with your hand).
Assembling the Brookies:
1. Prepare your 18x28cm rectangular springform pan by lining the bottom with parchment paper and buttering the sides.
2. Spread half of the brownie mixture into the pan, then add half of the cookie dough, little by little, spreading it evenly. Then add the remaining brownie mixture and, in the same way, add the rest of the cookie dough by placing pieces and pressing them down here and there.