Chop the dark chocolate into small, even pieces and place in a heatproof bowl. Measure milk, separate eggs and place yolks in a medium bowl. Keep cold cream chilled in the refrigerator until needed.

Make a tempered custard (chocolatier-style)

In a small saucepan, heat the milk over medium heat until it just begins to steam and small bubbles form at the edge (do not boil). While milk heats, whisk the egg yolks with the granulated sugar until pale and slightly thickened. Slowly pour about one-third of the hot milk into the yolk-sugar mixture, whisking continuously to temper the yolks. Return the tempered yolk mixture into the saucepan with the remaining milk and cook over low-medium heat, stirring constantly with a spatula, until the mixture thickens slightly and coats the back of a spoon (do not let it boil). Remove from heat immediately.

Incorporate chocolate

Pour the hot custard over the chopped chocolate. Let sit for 1 minute to melt, then whisk gently until smooth and glossy. Add the cubed butter and salt; whisk until butter is fully incorporated and the ganache-like base is silky. If any lumps remain, pass the mixture through a fine sieve into a clean bowl.

Cool base

Cover the chocolate base with plastic wrap, pressing the wrap onto the surface to prevent a skin, and let cool to room temperature (about 20–30 minutes). It should be fluid but not warm to the touch.

Whip cream for folding

Whip the 350 ml cold heavy cream to soft-medium peaks: start at low speed, increase to medium-high, and stop when the cream holds soft ridges but is still supple (do not overwhip).

Fold and set

Gently fold one-quarter of the whipped cream into the cooled chocolate base to lighten it (use a spatula and fold with large strokes). Fold in the remaining whipped cream in two additions until homogeneous and airy. Do not overmix. Portion the mousse into serving glasses or a single mold, smoothing the tops. Chill in the refrigerator for at least 4 hours, preferably overnight, to set.

Crème Chantilly (Whipped Cream)

Just before serving, whip 150 ml cold heavy cream with the icing sugar and vanilla extract to medium-stiff peaks. Keep chilled until ready to pipe or spoon onto mousse. If using a piping bag, fit with a plain or star tip and fill immediately.

Finish & Garnish

When mousse is fully set, spoon or pipe a generous dollop of Crème Chantilly on each portion. Sprinkle chocolate pearls over the top and dust with grated dark chocolate. Add a mint leaf for color if desired. Serve chilled.