Preheat the oven to 180°C (fan) / 200°C (conventional). Use the greaseproof paper to lift the frozen Biscoff slab out of the tin and return it to the freezer. Reline the baking tin ready for the brownie mixture.
Break the dark chocolate up into a bowl and add the butter. Microwave in 30-second increments, stirring after each, until completely smooth and melted. Set aside to cool.
150 grams 70% dark chocolate, 150 grams Unsalted butter
Vigorously whisk the eggs and sugar in a separate bowl for a couple of minutes until pale in colour, thick and foamy.
2 large Eggs, 200 grams Caster sugar
Stir the flour and cocoa powder together.
130 grams Self-raising flour, 20 grams Cocoa powder
Add the dry ingredients and the melted chocolate mixture into the whisked eggs. Fold to combine.