Prepare: Preheat your oven to 175°C (350°F). Line a 7x9" rectangular pan or an 8" square pan with some parchment paper, and create a parchment sling to make it easy to lift out the brownies once ready.Sift the flour, baking powder, and salt into a bowl and set aside.
Make the cookie crust: Add the cookies to your food processor and blitz for about 30 seconds until evenly finely ground. Add in the vegan butter and blend again until evenly combined and the mixture sticks together between your fingers. Press the cookie mixture into the base of the cake tin, and smooth it out with the back of a spoon, making sure it is compact. Set aside in the fridge while you prepare the brownie batter.
Melt the chocolate: Add the dark chocolate and vegan butter to a heat-resistant bowl. Place the bowl on top of a saucepan that is about ¼ ways filled with simmering water (bain-marie). Allow it to melt slowly. While the chocolate melts, continue with the next steps.
Make the brown sugar espresso cocoa syrup: To a medium bowl, add the brown sugar and hot espresso. Whisk for a minute until the sugar has dissolved into the hot liquid.Add the cocoa powder and whisk again until smooth. Set aside.
Whisk the aquafaba: Add the aquafaba to a large clean bowl or stand mixer and whisk with an electric mixer on high speed for 5 minutes until soft peaks form.Slowly add the granulated sugar, a couple of tablespoons at a time, and whisk for 30 seconds each time to combine the sugar into the aquafaba. Continue until all of the sugar has been incorporated, then whisk in the vanilla.
Combine the wet ingredients: Once the chocolate and butter have melted completely, stir them until smooth. Add them to the bowl with the brown sugar cocoa and espresso mixture and whisk to combine.
Mix the batter: Pour the wet ingredients into the bowl with the whipped aquafaba and carefully fold them through using a spatula.Add the dry ingredients to the bowl and carefully whisk until combined, and there are no visible pockets of flour.
Assemble: Pour half of the batter into the prepared brownie pan and smooth out the surface using a spatula. Top with 12 dollops of biscoff spread. Add the remaining brownie batter and gently smooth it out with a spatula. Arrange the remaining biscoff cookies on top of the batter.
Bake: Bake for 25-30 minutes. To check they're ready, insert a toothpick or skewer, it should come out with a little bit of batter and some moist crumbs.
Serving: Allow the brownies to cool completely for 30 minutes to an hour before cutting them into servings. For best results chill them in the fridge for 1-2 hours before slicing them with a sharp knife.
Storage: For a nice fudgy brownie consistency, store them in an airtight container on the counter for 3-4 days. When refrigerated, they will become quite solid. But they will stay fresh for longer! They keep in a sealed container in the fridge for over a week.
Notes
Vegan butter: Make sure you use vegan block butter NOT spreadable butter from a tub. I use Violife Block or Flora Plant Butter.
Aquafaba: The liquid part from tinned chickpeas (chickpea brine).
Dutch-processed cocoa: This is alkalized cocoa powder, check the package for acidity regulators such as potassium carbonates if you're unsure of your brand.
Some sugar in the US is not vegan, if in doubt use organic sugar.