Using a cold whisk attachment and a cold mixing bowl, beat the heavy cream and 2 tablespoons of sugar until soft peaks form. Refrigerate the whipped cream until needed.

Set up a double boiler with water heated over medium heat. Add chopped chocolate, 1/4 cup of water, unsalted butter, and vanilla extract into the double boiler.

Stir gently until the mixture is smooth and glossy.

In a small saucepan, whisk together egg yolks, remaining 1/4 cup of water, and 1/4 cup granulated sugar. Cook over medium-low heat and stir constantly until the mixture reaches 160°F.

Once at temperature, remove from heat and stir into the chocolate mixture until fully combined.

Cool the chocolate mixture in an ice bath for 5 to 7 minutes while stirring occasionally.

Gently fold the reserved whipped cream into the cooled chocolate mixture until smooth.

Divide the mousse into serving glasses and refrigerate for at least 4 hours before serving.