Pre-heat your oven to 160C Fan/350F/Gas Mark 4, and grease and line a 9" square traybake tin
Melt the butter and light brown soft sugar in a pan on a low heat until the butter is melted
Remove from the heat and add the white chocolate, leave it for a few minutes for the chocolate to melt, then stir together. Leave the mixture to cool for 5 minutes. Don't worry if it looks a little lumpy, it will go smooth again when you add the eggs
Transfer the mixture to a mixing bowl at this point if you like. Add the eggs and vanilla extract to the mixture, and whisk to combine
Add the plain flour, white chocolate chips and broken up Biscoff biscuits, and mix them in, then pour the mixture into the lined tin
Add small spoonfuls of the Biscoff spread all over. Use a butter knife to swirl it around a little bit - don't over mix.
Bake the blondies for 30-35 minutes, or until they are golden brown all over. Using a skewer inserted in the centre doesn’t give a clear indication that they are done because blondies are meant to be gooey and almost a little under cooked. Blondies are the white chocolate version of brownies and they are not like cake. They will also set more as they cool down
Leave to cool in the tin, then put them in the fridge for a couple of hours. Then remove from the tin and slice. Store in an airtight container in a cool place and eat within 3 days