Place seeds in a food processor and alternating between the grind and chop function, process the seeds until finally grinded. A liquid mixture will form. Strain the mixture into a bowl thru a fine mesh and discard the solids.
Place a small sauce pan over medium-low heat, add the pomegranate mixture with 1/3 cup of sugar. Simmer for 10-15 minutes, stirring occasionally. Mix corn starch with 2 tablespoons of water until corn starch dissolves, add it to the pomegranate mixture. Keep, simmering on low heat, the mixture will reduce in volume by 1/3.
In the meantime, in a small bowl, place gelatin and water, allow the gelatin to bloom, let it sit for 10 minutes.
Remove pomegranate sauce from heat, add the dissolved gelatin and stir well to combine. Set aside to cool for 15 minutes at room temperature. Stir occasionally to avoid jellification.
In the bowl of a stand mixer fitted with the wire attachment, place the remaining 1 1/2 cups of cream and 1 tablespoon of sugar and whisk until soft peaks form.
Once the pomegranate sauce has cooled to room temperature, add it to the cream and whisk until incorporated and the mixture is stiff. Don't over mix or you will curdle the cream. If you wish your mousse to be pinker, add in a few drops of pink gel food coloring at this point. To avoid over mixing, turn off the mixer, and gently mix the ingredients manually with a spatula.
Pour on top of the cooled brownie and level the top layer. Place in the fridge overnight, or for at least 6 hours.
Before serving, remove from the fridge, dust with cocoa powder, run a knife thru cold water and cut.