Fit your stand mixer with the whisk attachment, and place the mixing bowl in the fridge to chill while you prepare the base.

Set a large pot over medium-low heat and arrange a bowl on top to form a double boiler. Add the egg yolks and sugar, whisking steadily until the mixture thickens, becomes creamy, and increases in volume. Once it reaches that stage, add the chocolate and allow it to melt slowly, stirring until smooth, about 12 to 15 minutes.

Stir in a small splash of cream and a pinch of salt until fully blended, then take it off the heat and set aside.

Take your chilled bowl from the fridge and pour in the heavy cream, sugar, and vanilla. Whisk on high speed until the cream is fully whipped and holds its shape, about 10 to 12 minutes. This can also be done by hand, though it will take longer. Set aside about 1/4 cup for serving and return it to the fridge.

With the remaining whipped cream, gently fold in the melted chocolate mixture. Use slow, careful motions, lifting and folding rather than stirring, so you do not deflate the cream. An eight-shaped motion works well. Continue until the mixture is smooth and uniform, with no streaks remaining.

Divide the mousse evenly into four small or medium cups or ramekins. Place them in the fridge to set for at least 4 hours, or overnight.

To serve, finish each portion with the reserved whipped cream and a scattering of chocolate shavings made from the remaining chocolate.