Cut a piece of parchment paper so that it fits inside of an 8x8-inch square baking pan. Crease the edges in the shape of the square pan. Remove the parchment and place on a baking sheet. Spread the cookie butter in an even layer within the creased edges to form the layer that will go in between the brownie layers. Place in the freeze and let firm up while you make the batter.

Preheat the oven to 350˚F. Line the 8x8-inch baking pan with parchment paper, leaving the edges long enough to hang over the edges. Set aside.

Add salt, espresso powder, flour, and cocoa powder. Stir until just combined. Do no over mix!

Scoop half of the batter into the lined baking pan, spreading it into an even layer. Remove the cookie butter from the freezer and take off the parchment paper. Place the cookie butter on top of the brownie batter in the pan. Spread the remaining brownie batter on top of the cookie butter layer evenly.

Break up the cookies, if using, into pieces and lightly press them into the top of the batter.

Bake for 30 to 40 minutes, until the brownies are cooked through and just set. Use a toothpick inserted into the top layer to test. The toothpick should come out with wet crumbs, but not batter.

When finished, remove from the oven and sprinkle with fleur de sel, if using. Let cool completely before slicing.