Preheat your oven to 350°F (175°C). Grease a 9×13 inch baking pan or line it with parchment paper.

Melt the unsalted butter in a medium saucepan over low heat. Stir until shiny, about 2-3 minutes. Remove from heat to cool slightly.

Combine granulated sugar with the melted butter, then add eggs one at a time, whisking until glossy. Stir in vanilla extract.

In a separate bowl, sift together flour, cocoa powder, and salt. Gradually add to wet ingredients, stirring gently until just combined.

Fold in Biscoff spread and chocolate chips to create a marbled effect.

Pour brownie batter into the prepared pan, spreading evenly. Ensure uniform thickness.

Bake for 25-30 minutes. Check for doneness with a toothpick; it should come out with a few moist crumbs.

Let the brownies cool for at least 15 minutes before cutting into squares with a warm knife.