Preheat oven to 350 degrees and line an 8x8 pan with parchment paper.
Add Biscoff cookies to a Ziploc bag and crush till fine, or use a food processor. Melt 5 tablespoon butter in microwave. Combine the crushed and melted butter in a bowl until well blended. Press mixture evenly into the bottom of the lined baking pan.
In a medium bowl, combine flour, baking soda, and salt.
In a large, separate mixing bowl, add eggs, granulated sugar, brown sugar, and vanilla. Whisk together until well blended.
In a pot on the stove over low heat, add the butter and broken up pieces of chocolate. Stir frequently until completely melted and blended. Remove from heat to cool slightly for 1-2 minutes. Whisk the melted chocolate into egg mixture.
One cup at a time, add the dry ingredient mixture into the wet ingredients and mix until mostly smooth.
Pour brownie batter evenly over prepare cookie crust.
Heat the cookie butter in the microwave for about 15 and stir until smooth and thin. With a spoon, drop dollops evenly over the entire surface of brownie batter. Using a butter knife, gently swirl brownies in an "S": shape until you've reached your desired marbling effect.
Bake in oven for 23-35 minutes. Cook about 25 minutes for fudgier brownies and increase time for more cake like brownies.