Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan, or line it with parchment paper.

In a large mixing bowl, mix melted unsalted butter, brown sugar, and granulated sugar until smooth and glossy.

Add two large eggs one at a time, whisking until incorporated. Stir in vanilla extract.

In a separate bowl, whisk together all-purpose flour, baking powder, baking soda, and salt.

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

Gently fold in Biscoff cookie butter until the batter is smooth with swirls.

Incorporate the crushed Biscoff cookies into the batter, reserving some for topping.

Pour the batter into the prepared pan, smoothing the surface. Sprinkle reserved crushed cookies on top.

Bake in the oven for 25-30 minutes until lightly golden and a toothpick comes out with a few moist crumbs.

Sprinkle a pinch of sea salt over the top while warm. Let cool in the pan for about 15 minutes before transferring to a wire rack.

Allow to cool completely before cutting into squares.