• Preheat your oven to 350°F. Line a 9x9-inch baking pan with parchment paper and set aside.

Brown the Butter

• Brown the butter first. Add the butter to a cast iron skillet or saucepan over medium heat. As the butter melts, it will begin to foam and crackle. Stir frequently and watch closely. After several minutes, the milk solids at the bottom will turn golden brown and the butter will smell warm, nutty, and caramel-like. Remove from the heat immediately to prevent burning and allow it to cool slightly.
• If you're short on time, you can use melted butter instead!

Sift Dry Ingredients

• In a medium bowl, sift together the flour, baking powder, and salt. Set aside.

Mix the Wet Ingredients

• In a large mixing bowl, whisk together the browned butter and brown sugar until smooth.
• Add the Biscoff cookie butter, eggs, and egg yolk. Whisk until fully combined.
• Add the sourdough discard and vanilla extract. Whisk thoroughly until the discard is fully incorporated and the mixture is smooth.

Combine the Dry and Wet Ingredients

• Using a spatula, gently fold the dry ingredients into the wet ingredients until just combined. Do not overmix.
• Fold in half of the chopped white chocolate.
• Spread the batter evenly into the prepared baking pan. Sprinkle the remaining white chocolate over the top, followed by the crushed Biscoff cookies. I prefer to add the cookies after the blondies are done baking, because I like the crunch of the cookies on the blondies. Do whatever you prefer!

Bake the Blondies

• Bake for 25–28 minutes, or until the edges are golden brown and set. The center may still seem slightly underdone — this is completely normal for blondies. They continue to set as they cool, creating a soft, chewy, fudgy texture.
• Allow the blondies to cool completely before drizzling with extra melted Biscoff cookie butter (I microwave it for about 15-20 seconds) and slicing into squares.