To try this, freeze a layer of Biscoff and a layer of Nutella in similarly-sized squares on parchment paper. Once the sheets are frozen solid — which takes at least an hour — layer the spreads in a pan with your brownie batter in this order: 1/3 of the batter, Nutella, another 1/3 batter, Biscoff, and then a final layer of batter on top before baking.