Preheat the oven to 160℃ fan / 180℃/ 320℉ convection / 356℉.
Line a 20 x 20cm (8 x 8") square baking tin with baking paper / parchment.
In a ban marie (a heatproof bowl set over a pan of barely simmering water), melt the dark chocolate and butter. Cool.
Using a stand mixer fitted with the whisk attachment, or an electric whisk, whisk the eggs and caster sugar together for 3 minutes until light and tripled in volume.
Gently stir the cooled melted butter and chocolate into the fluffy beaten eggs and sugar.
Sift in the plain flour, salt, ground cinnamon and cocoa powder and stir with a spoon until just combined.
Gently fold in the chunks of biscuits and roughly chopped milk chocolate.
Pour the mixture into the lined tin and knock gently to release any air bubbles.
Dot teaspoons of the biscoff cookie butter / spread over the top of the brownie, then using a blunt knife like a butter knife, lightly drag the brownie mixture through the cookie butter to create a marble effect.
Scatter over 3-4 roughly broken biscoff biscuits.
Place in the preheated oven and bake for 20-23 minutes, until the brownie doesn't wobble in the middle, and a skewer inserted into the middle of the brownie comes out sticky not raw.
(If the skewer comes out very sticky and covered in runny batter, return to the oven for 2 minutes and test again.)
Leave to cool in the tin. Cool completely before cutting in to squares.
Store in an airtight container at room temperature for 5 days, or freeze for up to 1 month.