Preheat oven to 350°F (175°C) and grease a 9x9-inch baking pan or line it with parchment paper.
Melt unsalted butter and Biscoff cookie butter together in a medium saucepan over low heat, stirring until smooth.
Remove from heat and let cool for 5 minutes.
Whisk brown sugar into the butter mixture until smooth.
Add eggs and whisk until fully incorporated.
Stir in vanilla extract evenly.
Sift all-purpose flour, baking powder, and salt together in a large bowl.
Gradually fold the dry ingredients into the wet mixture until just combined to avoid overmixing.
Fold in white chocolate chips and crushed Biscoff cookies evenly.
Spread the batter evenly in the prepared baking pan.
Bake for 25 to 30 minutes until a toothpick inserted comes out clean or with few crumbs.
Cool in the pan on a wire rack for 10 minutes, then remove and cool completely before cutting.
Cut into squares and serve.