Place the chocolate in a heatproof large bowl. In a small saucepan over medium, bring the half-and-half, butter and salt to a simmer, occasionally swirling the pan. When the butter has fully melted, immediately pour the mixture over the chocolate. Let stand for 2 to 3 minutes to allow the chocolate to soften.

Whisk until the mixture is completely smooth. It should now be warm to the touch, not hot. If it is still hot, allow it to cool until warm, whisking occasionally. Add the egg yolks and whisk until incorporated. Cool to room temperature.

In the clean bowl of a stand mixer or in a clean large bowl if using a hand mixer, combine the egg whites, sugar and cream of tartar. Using the whisk attachment or a hand mixer, whip on medium until the whites are voluminous and hold slightly droopy peaks, 3 to 4 minutes.

Scoop about one-third of the whipped whites onto the chocolate mixture and, using a silicone spatula, fold until a few white streaks remain to lighten and loosen the base. Add the remaining whipped whites. Now using a whisk, gently fold until no streaks remain. The mousse will be soft and pourable and resemble melted ice cream.

Transfer the mousse to a serving dish. Cover and refrigerate until chilled and set, at least 4 hours or up to 12 hours.

Using a vegetable peeler, shave chocolate from the bar (begin shaving at an edge) over the mousse, allowing the curls and shavings to fall onto the surface, until covered to your liking. Serve or re-cover and refrigerate until ready to serve.