Melt the butter and 50g of the Biscoff spread, either in a saucepan or in a large bowl in the microwave. Don't let it get too warm. You want it to melt but it shouldn't be hot. Mix well.
Next, add the sugars. Stir them in well.
Add the egg and vanilla extract. Stir in.
Add the flour, along with any other dry ingredients (baking powder and salt if needed). Fold in until you have a glossy batter.
Break up each Lotus biscuit into about 8 pieces; chunks not crumbs. Heat the remainder of the Biscoff spread in the microwave for about 10–20 seconds until it is melted and runny, but not hot.
Add two thirds of the Lotus biscuit pieces to the batter. Fold them in, distributing them evenly through the mix.
Transfer the batter to the prepared baking pan and give it a gentle shake to level it out.
Pour the melted Biscoff spread over the batter and swirl it in, making sure you get it into the corners. I find the easiest way to do this is with the handle of a teaspoon.
Arrange the remainder of the pieces of Lotus biscuit on top of the batter. Gently press each one in.
Bake for 30–35 minutes, until a papery crust forms. The edges will have risen and become crinkly. The centre will still be soft, barely set, and will wobble if you (gently) shake the pan.
Allow to stand for 10–15 minutes on a cooling rack. The centre will carry on cooking and firm up as the blondie cools.
Cut the cool Lotus Biscoff blondies into squares to serve. I find a 4 x 4 grid is perfect.