01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - Melt butter in a medium saucepan over medium heat. Cook, stirring constantly, until butter foams and turns golden brown with a nutty aroma (4-5 minutes). Pour immediately into a large mixing bowl. Cool for 5 minutes.
03 - Whisk brown sugar and granulated sugar into the browned butter until well combined.
04 - Add eggs and vanilla extract. Whisk until mixture is smooth and glossy.
05 - Sift in flour, baking powder, and salt. Fold gently with a spatula until just combined—do not overmix.
06 - Spread batter evenly into the prepared baking pan.
07 - Warm Biscoff cookie butter in microwave for 15-20 seconds until pourable. Drizzle dollops over batter, then use a knife to gently swirl through the top.
08 - Sprinkle chopped Biscoff cookies evenly over the surface.
09 - Bake for 22-26 minutes until edges are golden and center is just set (toothpick should come out with moist crumbs).
10 - Cool completely in pan before lifting out and slicing into squares.