Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, line a 8" square tin with greaseproof baking paper. I grease the tin first with a little butter to help the paper stick to it
To make the cookie dough base, use a stand mixer with the paddle attachment for the best results, or you can do it by hand with a spoon, Mix the softened butter and light brown soft sugar together until fluffy and well combined
Add the milk and vanilla extract and mix well until combined
Then add the plain flour and salt, and mix until a dough forms
Finally mix in the chocolate chips until well distributed
Press the cookie dough into the bottom of the tin using your hands, try and make it an even layer
Put the cookie dough in the fridge while you make the brownies, or for at least 30 minutes
To make the brownie batter, 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 10 minutes
In a mixing bowl whisk together the eggs and light brown sugar until frothy, by hand with a whisk or with an electric whisk
Add the melted chocolate and butter mixture to the egg and sugar mixture, whisking constantly as you pour it in
Add the plain flour and cocoa powder, and whisk in gently
Then stir in the chocolate chips
Take the tin with the cookie dough in it out of the fridge and cover them with the Biscoff sandwich biscuits, you may need to chop some of the biscuits in half to fully cover the cookie dough
Pour the brownie batter over the top, then add spoonfuls of the Biscoff spread all over. Use a butter knife to swirl it around a little bit - don't over mix
Bake for 45-55 minutes, the brownie layer should not wobble (or only very slightly) when you gentle shake the tin. A cocktail stick or thin skewer inserted in the centre should come out with moist crumbs on it, but not be wet
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, they can also be frozen