Make sure to freeze the biscoff for 3 hours, otherwise it will be too soft and sink to the bottom and burn during baking.
Don't use more than 1 tbsp of biscoff for each muffin as it will be too heavy and will sink to the bottom.
You can substitute the brown sugar for granulated or caster sugar
It is normal for the biscuits on the top to go soft due to moisture. You can either add the biscuits just before eating or leave them off altogether.
These muffins will last for 2-3 days in an airtight container