Grease and line a 20cm x 20cm square baking tin with baking paper and set aside.
Place the biscuits into the Thermomix bowl and crush for 8 seconds, Speed 8. Set aside in a separate bowl.
Place the butter and peanut butter into the Thermomix bowl. Melt for 3 minutes, 100 degrees, Speed 2.
Add the icing sugar and crushed biscuits and mix for 10 seconds, Speed 3.
Press down firmly into the prepared baking tin and set aside.
In a clean and dry Thermomix bowl, melt the chocolate for 3 minutes, 80 degrees, Speed 2. Add the peanut butter and mix for 5 seconds, Speed 4.
Pour over the top of the slice and spread out evenly with a spoon. Refrigerate for 2 hours before cutting into slices.
Store in an airtight container in the fridge for up to 7 days or freeze for up to 3 months.