Preheat your oven to 350°F (175°C) and grease or line a 9x9-inch baking pan.

Melt the unsalted butter and Biscoff cookie butter in a medium saucepan over low heat.

Remove from heat and allow the mixture to cool slightly for about 5 minutes.

Whisk in the brown sugar until well combined and smooth.

Incorporate the eggs one at a time, whisking thoroughly after each addition.

Mix in the vanilla extract until well combined.

Sift together the all-purpose flour, baking powder, and salt in a separate bowl and fold into the wet mixture.

Gently fold in the white chocolate chips and crushed Biscoff cookies.

Spread the batter evenly in the prepared pan.

Bake for 25-30 minutes, checking for doneness with a toothpick.

Let cool in the pan for about 10 minutes before transferring to a wire rack.

Slice into squares and serve warm or at room temperature.