Preheat the oven to 190°C (170 fan/ 375F/ Gas 5) and line a 12 hole muffin tin with muffin cases.
Put the caster sugar, plain flour, bicarbonate of soda and salt in a large mixing bowl.
In a large jug whisk together the melted butter, softened biscoff spread, eggs and yogurt. Pour into the dry ingredients, add the vanilla extract and cinnamon and mix gently to combine.
Spoon half the muffin batter equally between the 12 muffin cases.
Place ½ teaspoon of biscoff spread into each muffin case.
Spoon in the remaining muffin batter on top and bake for 18 minutes.
When cool, place the biscoff spread for the topping in a piping bag with a large nozzle and pipe the biscoff spread onto the top of each muffin. Alternatively, you can just dollop a tablespoon of biscoff spread onto each muffin.
Top with a biscoff sandwich biscuit.