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.
1 cup dark brown sugar, 1/2 cup granulated sugar
Melt chocolate and butter over low heat or you can do this in the microwave at 30-second increments.
2 cups dark chocolate chips , 1/2 cup unsalted butter
Remove from heat and pour into the bowl of your mixer with the sugars and begin mixing on low. Add eggs, vanilla, and salt.
4 eggs, 2 teaspoons vanilla extract, 1/2 teaspoon salt
Add flour, 1/2 cup at a time, and mix until fully incorporated.
1 cup all-purpose flour
Pour batter into prepared pan.
In a microwave-safe bowl, add Biscoff and microwave for 15-30 seconds.
1/2 cup Biscoff spread
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.