Cut a piece of parchment paper so that it fits into your an 8×8 square baking pan, leaving some hanging over the edges. Place it in the pan and fold the parchment that is overhanging to form a crease in the shape of the square pan.
Remove the parchment from the pan and place it on a flat surface like a cookie sheet, cutting board, or cooling rack. Spread the cookie butter onto the parchment in the shape of the square pan, so that it will fit into the baking pan in one piece. Freeze while you make the brownies.
Preheat the oven to 350°F and line with parchment paper. Leave parchment paper hanging over the sides of the pan so that you can easily remove the brownies from the pan later.
Mix the cocoa powder, salt, and flour in a bowl and set aside.
Melt the butter in the microwave or in a small pan over the stove. Transfer the melted butter to a medium sized bowl.
Add in both the sugars and use a whisk to whisk the butter and sugars together. Keep whisking until the mixture resembles a paste that pulls away cleanly from the side of the bowl when you mix it.
Add the eggs and vanilla and mix again.
Pour the dry ingredients into the bowl and mix with the whisk until just combined (do not over mix!).
Pour half of the batter into the lined baking pan, then take the cookie butter out of the freezer, peel it from the parchment (it should come off easily and in one piece*), and place the whole piece on top of the brownie batter.
Pour the remaining brownie batter on top of the cookie butter into the pan and smooth into an even layer.
Line the top of the brownies with Biscoff Cookies, pushing them into the batter slightly.
Bake for 30-40 minutes*. Begin checking the brownies at 30 minutes by inserting a toothpick in the middle. The top of the brownies should be crinkly and the toothpick should not come out clean. The brownies are ready when the toothpick has a few wet crumbs, but not wet batter.
Let the brownies cool completely before slicing. Enjoy!