Preheat oven to 325 °F (163 °C) and line a 9x13-inch baking pan with parchment paper or aluminum foil, leaving at least a 1-inch overhang on the sides. Spray with cooking spray.
Place brown sugar and granulated sugar into the bowl of your mixer and set aside.
Melt chocolate and butter over low heat or you can do this in the microwave at 30-second increments.
Remove from heat and pour into the bowl of your mixer with the sugars and begin mixing on low. Add eggs, vanilla, and salt.
Add flour, 1/2 cup at a time, and mix until fully incorporated.
Pour batter into prepared pan.
In a microwave-safe bowl, add Biscoff and microwave for 15-30 seconds.
Pour over the pan in 3 strips lengthwise. Run a butter knife through the batter the opposite direction (widthwise) to create swirls.
Bake 35-40 minutes until a toothpick inserted in the center comes out clean.