Preheat the oven to 170°C. Grease a 6-inch loaf tin and line with baking paper and set aside.
Make the egg replacer according to the packaging instructions and set aside.
In a separate bowl, mix the caster sugar, melted vegan spread, vanilla extract, cocoa powder, salt and baking powder, then stir in the egg substitute.
Once combined, fold in the flour, then add in melted dark chocolate and mix until combined.
Transfer the mixture into the tin.
Place the Biscoff spread onto a small microwave-safe dish and microwave for 10-20 seconds or until runny. Pour the Biscoff spread over the batter and drag a knife through the mixture a few times to marble the mixture. Level the top with a spatula and bake for around 15-20 minutes.
Cool completely before cutting into squares. These brownies taste delicious on the day they're made and even better the following day!