Pre-heat your oven to 180C/160C Fan/350F/Gas Mark 4 and grease and line a 12" x 9" traybake tin
Melt the dark chocolate and butter together in a pan on a low heat. When fully melted, stir together and remove from the heat. Let it cool for 5 minutes
In a bowl whisk together the eggs and light brown sugar until frothy
Add the chocolate mixture to the egg mixture, whisking constantly as you pour it in
Add the plain flour, cocoa powder and baking powder. Whisk in until you can't see any flour
Pour the batter into the lined traybake tin and add blobs of the Biscoff spread all over. Use a butter knife to swirl it around a little bit - don't over mix
Bake for 20 minutes
Take the brownies out of the oven, press the Lotus biscuits into the batter, then put them back in the oven for 10 minutes
Let the brownies cool completely in the tin before removing and slicing up
Store in an airtight container in a cool place and eat within 3 days
Video
Notes
If you would like to use a 8" or 9" square tin instead, reduce the recipe by one third. So you will need:
235g Dark chocolate
165g Butter
165g Light brown sugar
2 Eggs
½ tsp Baking powder
65g Plain flour
20g Cocoa powder
100g Biscoff spread
9 Lotus biscuits
They will also need less time to bake, depending on your oven, I'd suggest 20-25 minutes. Or until a toothpick inserted comes out with several moist crumbs on, but is not wet.