Preheat your fan oven to 160˚C (325˚F) or 180˚C (350˚F) if you're not using a fan oven. Grease and line with baking paper an 8x8 inch (20x20 xm) baking pan. Make sure all the sides are covered with baking paper, as well.
Using a food processor grind your cookies. In a separate bowl, combine them with melted butter. Press the mixture into the bottom of the pan. Bake for 5-6 minutes then set aside to cool.
In a medium sized bowl, sift and combine flour, cocoa powder and salt. Set aside.
Place chopped chocolate (or chocolate chips) and diced butter into a heatproof bowl and melt over a bain-marie (a pot of simmering water). Make sure the bowl doesn't actually touch boiling water. Stir occasionally with a silicone spatula, so it combines and melts evenly. Once it's done, set aside to cool a little bit.
In a bowl of your stand mixer fitted with the whisk attachment, mix together eggs and both sugars, until pale, fluffy and doubled in volume. You can't really overmix this, so let it beat on high for at least 5 minutes.
Then, set your mixer to low speed and slowly, in a steady stream, pour in your melted chocolate-butter mixture. Add the vanilla and mix until everything is combined.
Add your dry ingredients and mix on low, just until combined.
Pour the brownie batter into a prepared baking pan with cookie crust.
Melt cookie butter in the microwave or over a bain-marie (a pot of simmering water) to get a creamy consistency. Using a spoon, dollop blobs of cookie butter on top of the brownie batter. Then, using a skewer or a butter knife, swirl the cookie butter. (Put more than you think, I actually didn't put enough. :D)
Bake in a preheated oven for about 35 minutes. Half way through the baking, carefully drop your pan on the counter a couple of times to deflate it and create an even top layer.
Once they're done, place the pan on a cooling rack and let the brownies cool completely before cutting into it.
If you don't have a stand mixer, you can use an electric hand mixer.
If you still don’t own one, I advise you to buy a kitchen scale – they’re cheap and easy to use, and always guarantee the same results in baking!
Not all ovens are the same, check the brownie at 30 minute mark, then adjust the time accordingly. Don't overbake it!