1. Melt Chocolate Base: In a large heat-proof bowl set over a pot of simmering water (double boiler) or in the microwave in 30-second bursts, melt the chopped chocolate and butter together until completely smooth. Remove from heat and let it cool slightly (about 5 minutes).

2. Incorporate Yolks: Whisk the 3 egg yolks into the slightly cooled chocolate mixture one at a time. Stir in the vanilla extract and salt. The mixture will look glossy and thick.

3. Whip Egg Whites: In a separate clean, dry bowl (or stand mixer), beat the 3 egg whites and cream of tartar on medium-high speed until soft peaks form.

4. Add Sugar: Gradually rain in the granulated sugar while continuing to beat on high speed until stiff, glossy peaks form. (The peaks should stand straight up when you lift the beater).

5. Lighten the Base: Take about ¼ of the whipped egg whites and whisk them vigorously into the chocolate mixture to loosen it up.

6. Fold (The Technique): Gently fold the remaining egg whites into the chocolate mixture using a rubber spatula. Use a ‘cut down the middle and scoop up’ motion.

7. Be gentle! You want to keep as much air in the batter as possible. Stop folding as soon as no white streaks remain.

8. Chill: Spoon the mousse into individual serving glasses or ramekins.

9. Refrigerate for at least 4 hours (or overnight) to allow the mousse to set and the flavors to deepen.

10. Serve: Top with a dollop of whipped cream and chocolate shavings before serving.