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

Melt the butter and Biscoff cookie butter in a saucepan over low heat until smooth, about 3-5 minutes.

Remove from heat and whisk in brown sugar until smooth and glossy after cooling for 5 minutes.

Add eggs one at a time, whisking until fully combined, then add vanilla extract.

In a separate bowl, whisk together flour, baking powder, and salt.

Fold the dry ingredients into the wet mixture until just combined.

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

Pour the batter into the prepared pan and spread evenly.

Bake for 25-30 minutes or until a toothpick comes out clean.

Allow to cool for 10 minutes in the pan before transferring to a wire rack to cool completely.