Crush the biscuits by placing in a ziplock bag and crushing with a rolling pin or pulsing in a food processor .
Add the butter and Biscoff spread to a mixing bowl and microwave for 40 seconds. Mix well until smooth.
Add the icing sugar and Biscoff cookie crumbs and mix until combined.
Spread the cookie bar mix into a lined baking pan and level.
Put the chocolate chips and Biscoff spread in a mixing bowl and heat in microwave for 30 second bursts, stirring in between, until melted and smooth.
Pour the chocolate over the cookie base and put in the fridge to set for at least 90 minutes or overnight.
Slice the bars using a large knife (it helps to run hot water over the blade). You can slice into 16 squares or even smaller - these bars are very rich! Drizzle with a little Biscoff spread if you like.
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Notes
These bars will keep well for up to a week stored at room temperature in an airtight container or in the fridge if it is warm.
Wrap cookie bars in cling film or aluminium foil and store them in a suitable container. Thaw before serving.