Prepare your brownies in a 9-inch circular pan as directed. Set Aside.
In a microwave safe bowl or in a small saucepan, heat up your chocolate until melted. Let cool slightly, before whisking through your egg yolks. Set aside.
In a small saucepan, add 200 mls (3/4 cup + 1 tablespoon) of heavy cream and heat on low. Once it begins to simmer, add the gelatine powder and whisk well. Remove from the heat and let cool slightly, before beating into the chocolate mixture, until combined.
Whisk the remaining cream until stiff peaks form. Set that aside, and in another bowl, beat your egg whites until stiff peaks form. Slowly, add the sweetener until fully mixed in and glossy.
Fold the whipped cream into the chocolate mixture, followed by the egg white mixture. Pour over the brownie base and refrigerate for at least 6 hours, or overnight, to firm up.
* You can replace this with one sheet gelatine.** You want the sweetener to have a superfine texture. Blend your monk fruit or erythritol until fine, but not powdered. Keto Chocolate Mousse Pie must always be refrigerated.