Brown the butter. Pre-heat oven to 170°C (340°F). In a small saucepan over medium heat, melt your butter. Continue to cook until the butter is golden brown and has a nutty fragrance. This takes about 4-5 minutes. Note that the butter turns very quickly from golden to burnt, so keep an eye on it. Remove from the heat, pour into a large mixing bowl, and leave at room temperature cool for ten minutes.
Prepare your pan. Line a 23cm (9-inch) square baking pan with parchment paper. Leave an overhang on the sides to make it easy to lift the blondies out after they are baked.
Mix sugar into the butter. Into the bowl with the brown butter, add brown sugar and using hand-held electric beaters, to beat on high until well mixed in.
Add eggs and Biscoff spread. Add the 70g/¼ cup of the Biscoff spread, two eggs, one egg yolk and vanilla extract into the mixing bowl and, using the hand-held electric beaters on high speed, beat until the mixture is visibly paler and thicker. This can take 3 to 4 minutes to achieve. The length of time not only cools the butter down and aerates the eggs. When you lift up the beaters, the mixture should ribbon off.
Add dry ingredients. Sift into this mixture the flour, baking powder and salt and fold through using a rubber spatula until a few flour streaks remain.
Add biscoff biscuits and chocolate. Break or chop half of the biscuits (8 biscuits) into small chunks (not crumbs). Add them and the white chocolate into the mixture and fold in the white chocolate until just combined.
Spoon batter into the prepared baking tray and level with an offset spatula. Bake. Bake for 15 minutes, remove the blondies from the oven and drizzle with remaining Biscoff spread. (Warm the spread in the microwave for ten seconds to loosen it.) Break the remaining biscuits into halves, place them over the top of the blondie square, and press gently into the blondies. Return to the oven for 15 -20 minutes or until lightly browned on top, and the sides are set, and a toothpick inserted into the centre comes out with a few moist crumbs. Remove from the oven and let sit in the pan for ten minutes. Using the paper, lift the blondie square out onto a wire rack and leave to cool completely. Use a sharp, hot knife, to cut into 16 neat squares and serve immediately.Serving suggestion. Drizzle with more Biscoff spread and serve with a scoop of ice cream.
Step-by-Step Photos: My blog post includes helpful step-by-step photos to help guide you through making this recipe.How to store: Store blondies in an airtight container at room temperature for up to 4 days or in the refrigerator for up to 1 week.Freeze: Wrap blondies individually in plastic wrap and freeze for up to 3 months. Thaw at room temperature.TIP 1: Brown butter bits. Make sure to scrape all those brown butter bits from the bottom of the saucepan- that's where the flavour is held.TIP 2: Paper overhang. Set yourself up for success and make sure to use the baking paper as a sling.TIP 3: Warm the Biscoff Spread. Just like with peanut butter, biscoff spread is quite thick. Warm it for ten-20 seconds in the microwave to loosen it enough to stir through batter or drizzle over the top of blondies.TIP 4: Glossy, crackly thin top. The sure-fire way to get that golden papery thin top is to whip the eggs until pale and thick so that they can work their magic once baked.Tips 5: Don't overcook. You want a slight jiggle in the centre of the blondie for that soft, fudgy interior. Bake for longer for a more ‘cakey’ texture.