Preheat your oven to 350°F (175°C) and prepare a 9x9-inch baking pan by greasing it or lining it with parchment paper.

Melt the unsalted butter and Biscoff cookie butter in a saucepan over low heat, stirring until smooth, for 3-5 minutes. Cool slightly.

Stir in brown sugar, then whisk in the eggs one at a time, followed by vanilla extract, until smooth.

In a separate bowl, sift together flour, baking powder, and salt. Fold the dry ingredients into the wet mixture carefully.

Gently fold in white chocolate chips and crushed Biscoff cookies until evenly distributed.

Pour the batter into the prepared pan and spread evenly. Tap the pan lightly to settle the batter.

Bake for 25-30 minutes until a toothpick comes out clean or with a few moist crumbs.

Cool for 10 minutes in the pan, then transfer to a wire rack to cool completely before cutting into squares.