Preheat the oven to 350°F (180°C). Spray a 9x13 baking dish with non-stick spray and line it with parchment paper so that there is overhang on either side.
Start off by adding 3 tablespoons of ground flaxseeds to a bowl and then add ½ cup (120ml) hot water from the kettle and mix it in. Leave it to thicken while you prepare the other ingredients. These are your flax eggs.
Sift all purpose flour and cocoa powder into a mixing bowl and add baking powder and salt. Mix together.
Add vegan butter and ½ cup chocolate chips to a microwave safe bowl and microwave in 30-second intervals, bringing it out to stir every 30-seconds until melted.
Add the flax eggs, white granulated sugar and vanilla extract to the melted vegan butter and chocolate and whisk it in.
Add the wet ingredients to the dry and mix it into a thick brownie batter.
Add the cup of chocolate chips and 20 chopped biscoff cookies and mix them into the batter.
Transfer the batter to your prepared 9x13 baking dish and smooth down.
Add biscoff cookie butter to a microwave safe bowl and microwave it in 15-second intervals, bringing it out to stir every 15-seconds until melted.
Use a teaspoon to drizzle the melted biscoff cookie butter over the top of the brownies.
Place into the oven and bake for 30 minutes until the top of the brownies looks dry and shiny.
Let them cool in the pan for 30 minutes before using the parchment paper overhang to lift them out of the dish and place them onto a wooden cutting board.
Let them cool completely (ideally in the fridge, see notes) before cutting into squares.