Preheat oven to 350°F (175°C) and prepare a 9x13 inch baking pan by greasing or lining with parchment paper.

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

Add eggs one at a time, whisking fully before adding the next. Stir in vanilla extract.

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

Fold dry ingredients into the wet mixture until just combined.

Gently fold in Biscoff cookie butter, aiming for a marbled effect.

Fold in crushed Biscoff cookies, saving some for topping.

Pour the batter into the prepared pan and top with reserved cookie crumbs. Bake for 25-30 minutes.

Sprinkle sea salt over the warm blondies after baking. Cool in the pan for 15 minutes before transferring to a wire rack to cool completely.

Slice into squares and enjoy.