If you've followed the recipe correctly, the two main reasons your Biscoff Blondies may come out cakey would be either over-mixing the batter or over-baking the blondies. You'll want them to be mostly baked, but just slightly under since they will continue to set as they cool. If they are baked through when you remove them from the oven, because they will continue to bake in the hot pan, they may be over-baked by the time they have fully cooled.