Beat 1 cup of heavy whipping cream using a hand-held mixer for 30 seconds on low speed. Then, increase the speed to high and beat until the cream holds stiff peaks. Do not overmix! Set aside.
Pour water into the bottom of a saucepan set over medium-high heat.
Add sugar and stir occasionally until it comes to a boil.
Pour egg yolks into a mixing bowl and whisk to break up.
Continue whisking while slowly drizzling in the sugar syrup
Pour the remaining ½ cup of heavy whipping cream into the saucepan and set it back on the stove on low heat.
Stir in the egg mixture and continue to stir for at least 3 minutes until heated through.
You may heat until thickened, if desired.
Pour through a fine mesh sieve that is set over your bowl of melted semi-sweet chocolate.
Discard anything that remains in the sieve.
Stir the chocolate until glossy and smooth.
Whisk ⅔ cup of the whipping cream into the chocolate.
Fold another ⅔ cup of whipped cream into the chocolate, then fold in the remaining whipped cream.
Cover and chill until the mousse thickens.
Spoon or pipe into bowls or chocolate cups.
Let sit at room temperature for at least 15 minutes before serving.