Preheat your oven to 180°C (gas mark 4, 350°F) and line a 9″ square baking tin with parchment paper.
Add butter to a large mixing bowl and heat in the microwave until melted. Leave the butter to cool down for 1 to 2 minutes, add the caster sugar and light brown sugar and mix until combined.
Add eggs and vanilla to the mixing bowl and mix together until combined.
Pour plain flour into the mixing bowl and mix together until a batter has formed.
Add the chopped white chocolate and crushed Biscoff biscuits into the mixing bowl and mix together until evenly distributed.
Pour the blondie batter into a lined baking tin and spread evenly using a spatula.
Place spoonfuls of Biscoff spread on top of the blondie batter and swirl through the batter (I used a skewer to create a marble pattern). Then place 8 Biscoff biscuits on top.
Bake in the oven for 20 to 25 minutes or until golden and slightly wobbly in the middle.
Leave the blondies to cool down and firm-up before cutting into slices. You can place it in the fridge to help the blondies set with a fudgy texture.