Preheat the oven to 170 C (160 fan). Lightly grease and line the baking tin with baking paper. Set aside.
Break roughly half of the biscoff biscuits into smaller pieces, reserve 8 whole biscuits for the topping (or break them all if you would like).
In a bowl of an electric mixer, mix the softened butter and both sugars until pale and fluffy, about 3 minutes. Add vanilla and eggs, one at a time, scraping the bowl well after each addition. Mix until combined, about 1- 2 minutes.
Add flour and salt and using a spatula or a wooden spoon, gently mix all the ingredients together, until just combined. Fold in chopped white chocolate and crushed biscoff biscuits. Transfer the batter into lined baking tin and smooth the top using the back of the spoon or a spatula.
Place the biscoff spread in a microwave for about 15 seconds, then add small blobs of biscoff spread on top of the blondie batter. Use a toothpick or a chopstick to gently swirl the spread into the rest of the batter.
Top the blondies with remaining 8 biscuits and bake in the oven for about 25-30 minutes, until golden, the middle should have a slight wobble to it. It's super important NOT TO OVER-BAKE these blondies. When inserting toothpick in the middle, it should come out with some crumb attached to it.
Let them cool in the tin for about 20 minutes, then transfer to the fridge for at least an hour to two hours. This will help the blondies to firm up and solidify. Do not attempt to cut the blondies while they are still hot, as they will be to gooey!
Once cooled completely, remove from the fridge and the baking tin and slice into 9 equal pieces. Enjoy!
Baking times may vary, depending on the oven. It took 30 minutes for my blondies to bake in the fan assisted electric oven.
DO NOT OVER- BAKE these bars. If they come out too cake-like, that means they are over- baked. Bake for shorter period of time next time! The texture should be gooey and slightly under-baked.
It is better to under-bake the blondies and allow them to firm up in the fridge.