Preheat the oven to 160C/325F. Prepare an 8 x 8 baking tin by lining with parchment.
Melt the butter in a small saucepan.
½ cup butter
Whilst the butter is melting, mix together the sugars and cocoa powder in a medium bowl until fully combined. Add the melted butter and whisk until combined. Mix in the salt and vanilla extract. Don't worry if the mixture looks grainy as it'll come together later.
¾ cup granulated sugar, ½ cup soft brown sugar, ¾ cup cocoa powder, ½ teaspoon sea salt, 2 teaspoons vanilla extract
Crack the eggs into the batter one at a time and mix each one until fully incorporated before adding the next. Vigorously stir in the flour until it starts to look glossy and come together into a thick batter (about 40-50 strokes). Fold in the chopped dark chocolate.
2 eggs, ¾ cup all purpose flour, ½ cup dark chocolate
Spread just under half the batter evenly across the bottom of the prepared baking tin with a spatula. Create little dips over the batter and place a heaping teaspoon of Biscoff spread into each one. (about ⅔ cup)
1 cup smooth biscoff spread
Dollop the rest of the brownie batter over the top of the mix between the biscoff dollops and spread out with a spatula ensuring not to disturb the biscoff too much. Top the brownie with more dollops of biscoff (⅓ cup) and swirl into the top with the back of a spoon. (see notes) Crumble the biscoff biscuits over the top.
4 biscoff biscuits
Bake at 160C/325F for 25-35 minutes, there will still be a slight wobble when it is ready. (30 minutes was perfect for me in my oven)