Preheat the oven to 180°C (350°F) and line a 9'x9' baking tin with baking paper.
Melt the butter and dark chocolate together.
Mix the eggs, sugar, brown sugar and vanilla extract together until the mixture becomes a light golden colour.
Mix in the chocolate mixture.
Stir in the flour and cocoa until just combined.
Stir through the chocolate chips if you are adding them.
Spread half of the mixture into the prepared pan in an even layer.
Spread over 150g of the Biscoff spread. If your spread is quite stiff, through it in the microwave for 10 seconds to melt it slightly.
Cover with the remaining brownie batter and then dollop the remaining Biscoff spread over top and swirl it through with a knife.
Place in the oven to bake for 20-25 minutes. The top and sides will be set and the middle will have crumbs attached to a skewer inserted but should not be wet.