Preheat your oven to 180C/160C Fan and line a 9x9" square tin with parchment paper.
In a large bowl, add your melted butter and sugars and beat till smooth.
Add in the eggs and vanilla and beat again till smooth.
Add in the flour and cornflour and beat till a thick blondie mixture is made - it really doesn't take long to create this mixture!
Add in your white chocolate and fold through!
Pour the mixture into the tin, and spread. Dollop on the biscoff spread and lightly swirl through the blondie mixture.
Sprinkle on the chopped biscuits!
Bake the Blondies in the oven for 25-30+ minutes, or until there is a slight wobble in the middle. (See notes below)
Leave to cool in the tin, or on a wire rack.
As an optional extra, you can 'set' your blondies in the fridge for an hour or so to help with the texture! ENJOY!
I've updated the butter quanitity to be 200g rather than 250g as it was occasionally being greasy for readers - if you liked it at the 250g quantitiy you can still use it!
The bake time can vary - please make sure your oven is on the correct setting, and check from 22+minutes and onwards till they are baked! These took 25 minutes in my fan oven.
If they turn out cakey, they are over baked - that's all! Bake for a little less time next time.
One top tip is if they've been in for the 25-30 minutes, and you're still not sure, let them cool outside of the oven fully and then put them in the fridge for at least an hour to 'set'. It can firm them right up!