In the bowl of a stand mixer fitted with the whisk attachment, beat cold cream and vanilla, slowly increasing mixer speed to medium-high, until stiff peaks form, about 2 minutes. Transfer to a medium bowl; cover and refrigerate until ready to use. Clean bowl of stand mixer and whisk.
In the top of a double boiler, place chocolate and butter. Cook over simmering water, stirring occasionally, until melted and smooth. Let stand until ready to use.
In a large heatproof bowl over simmering water, place egg yolks and ¼ cup (50 grams) sugar. Cook, whisking frequently, until thickened and ribbon-consistency and an instant-read thermometer registers 158°F (70°C). Remove from heat, and set aside.
In the heatproof bowl of a stand mixer, whisk together egg whites and remaining ½ cup (100 grams) sugar by hand. Place bowl over a saucepan of simmering water. Cook, whisking frequently, until an instant-read thermometer registers 120°F (49°C) to 130°F (54°C).
Carefully return bowl to stand mixer. Using the whisk attachment, beat at medium-high speed until stiff peaks form.
Using a balloon whisk, fold egg white mixture into egg yolk mixture in three additions alternately with chocolate mixture, beginning and ending with egg white mixture. Fold in whipped cream in two additions. Use immediately.
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/baking-school-depth-chocolate-mousse-cake/