In a pan with boiling water, place a heat-proof bowl on top and add the bittersweet chocolate and butter. Stir to melt the chocolate au bain-marie. Once the chocolate is melted and smooth, remove the bowl, and clean the bottom of the bowl. Add the sugar and stir. Then add, in order, the eggs, unsweetened cacao powder, all-purpose flour, and salt. Whisk to incorporate all the ingredients well. Pour the brownie batter into the baking sheet. Place a layer of Oreo cookies on top and set aside.