Preheat your oven to 350F. Line a 9x9 inch baking pan with parchment paper, leaving overhang on two sides for easy removal.
In a heatproof bowl set over a pot of simmering water, melt the butter and dark chocolate together, stirring frequently until completely smooth. Remove from heat and let cool for 5 minutes.
Whisk the granulated sugar into the chocolate mixture until combined. Add the eggs one at a time, whisking well after each, then stir in the vanilla extract. The batter should look thick and glossy.
Sift the flour, cocoa powder, and salt over the batter. Fold gently with a spatula until just combined - do not overmix.
Pour the brownie batter into the prepared pan and spread evenly. Warm the Biscoff cookie butter in the microwave for 15-20 seconds until pourable, then dollop large spoonfuls over the surface of the batter.
Use a toothpick or skewer to swirl the cookie butter through the brownie batter in a decorative pattern. Scatter the crushed Biscoff cookies over the top, pressing them in lightly.
Bake for 28-32 minutes until the edges are set and slightly pulling away from the pan, but the center still has a slight jiggle. Let cool completely in the pan before lifting out and slicing into squares.