Preheat the oven to 350ºF (177ºC) and grease a 9x13-inch (23 x 33cm) baking pan with cooking spray.
Prepare the brownie batter. Combine the brownie mix, eggs, melted butter, and milk in a large mixing bowl with a hand or stand mixer until well combined, about 1-2 minutes.
Add chocolate. Fold the milk chocolate chips into the batter until evenly distributed.
Base layer. Spread 2 cups (473mL) of the batter into the bottom of the prepared pan in an even layer.
Cookie butter layer. Warm the Biscoff cookie butter spread in the microwave until pourable. Pour it as evenly as possible across the top of the brownie batter.
Gently even out the cookie butter layer over the brownie layer using an offset spatula or a butter knife.
Add the remaining brownie batter. Carefully add the remaining brownie batter on top of the cookie butter layer, starting at the edges, then to the middle, and carefully spread it into an even layer. Work carefully to avoid blending with the cookie butter layer.
Add the cookies. Add the roughly chopped cookies on top, pressing them into the batter very gently.
Bake. Place in the oven and bake for 33-38 minutes or until a toothpick inserted in the center of the brownies comes out mostly clean.
Slice. Allow the brownies to cool completely before cutting into squares and serving.
Video
Notes
Storing leftovers: Allow the brownies to cool completely, then store them in an airtight container for up to 3 days at room temperature, or 5 days in the refrigerator. To freeze: Wrap individually in plastic wrap, then add to a freezer bag. Freeze for up to 3 months, and thaw (or heat) before serving.