Begin by melting 8 oz (250g) of dark chocolate with 3 tbsp brandy in a heatproof bowl set over a pan of hot water, stirring gently until completely melted, then allow it to cool.
While the chocolate cools, prepare the gelatin by sprinkling 2 tbsp powdered gelatin over 3 tablespoons of cold water in a separate heatproof bowl and let it sit for about 10 minutes until it becomes spongy.
Once spongy, place the bowl in a pan of hot water and heat gently, stirring occasionally, until the gelatin is completely dissolved and clear.
In a large heatproof bowl, combine 4 large eggs, 2 large egg yolks, and ⅓ cup granulated sugar. Place this bowl over a saucepan of simmering water and whisk continuously with a handheld electric mixer until the mixture becomes very thick and mousse-like in consistency.
This step is essential for achieving the proper texture. Once the egg mixture reaches the desired thickness, whisk in the dissolved gelatin until fully incorporated.
Fold ⅔ cup of whipped cream into the cooled chocolate until no streaks remain, then carefully fold this chocolate mixture into the egg and gelatin mixture using a spatula to maintain the airiness.
Pour the mousse gently into a glass serving bowl, cover it, and refrigerate until completely set, which typically takes several hours or overnight.
To serve, decorate the top with piped rosettes of ⅔ cup heavy or whipping cream, whipped until stiff, and scatter chocolate curls over the surface. Present chilled.
