Preheat oven to 350F. Line an 8x8 inch baking pan with parchment paper with slight overhang on each side. Set aside.
In a small bowl, whisk flour, baking powder, baking soda and salt. Set aside.
In a large bowl, beat brown sugar and melted butter until thoroughly combined.
Add the egg and egg yolk and stir into the butter mixture until incorporated.
Then add vanilla and Biscoff cookie butter. Continue to beat until smooth and incorporated.
Using a spatula, fold your flour mixture into your cookie butter mixture a third at a time. Do not over mix.
Fold in white chocolate chips. Note that the batter is going to be thick and sticky.
Transfer and evenly spread the batter into your prepared pan and bake for 15 minutes or just until they begin to set at the edges and peel away from the sides of the pan.
Allow to cool completely before cutting into squares.
Video
Notes
I use light brown or demerara sugar when making Biscoff blondies but you can use regular brown sugar as well.
When you take out the blondies from the oven, they will look underdone and when you touch the top, it will be very soft. This is OK. They will continue to set - but remain soft and fudgy - as they cool.
If the batter is still very runny in the middle but getting brown on the edges, cover loosely with aluminum foil and bake some more.
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