Place 4 cups of water in a 3-quart saucepan and bring to a gentle simmer over medium heat. In a stainless-steel mixing bowl large enough to nest in the saucepan without touching the water, combine chocolate and heavy cream, set it over the saucepan, and, stirring occasionally with a silicone spatula, cook until the chocolate has melted. Stir until homogenous, then remove the bowl from the saucepan and set on a heatproof work surface.
Place 4 cups of water in a 3-quart saucepan and bring to a gentle simmer over medium heat. In a stainless-steel mixing bowl large enough to nest in the saucepan without touching the water, combine chocolate and heavy cream, set it over the saucepan, and, stirring occasionally with a silicone spatula, cook until the chocolate has melted. Stir until homogenous, then remove the bowl from the saucepan and set on a heatproof work surface.

Whisk the 3 egg yolks into the chocolate-cream mixture until well combined (the chocolate mixture should be warm to the touch, but not hot). Set aside.
Whisk the 3 egg yolks into the chocolate-cream mixture until well combined (the chocolate mixture should be warm to the touch, but not hot). Set aside.

Meanwhile, in a the bowl of a stand mixer fitted with the whisk attachment, combine egg whites, kosher salt, espresso powder, and cream of tartar. Whisk on medium speed until egg whites begin to foam, then, with mixer continuing to run at medium speed, slowly and steadily pour in the sugar. Continue to whisk until meringue is glossy and reaches medium-stiff peaks, about 8 minutes. Set aside.
Meanwhile, in a the bowl of a stand mixer fitted with the whisk attachment, combine egg whites, kosher salt, espresso powder, and cream of tartar. Whisk on medium speed until egg whites begin to foam, then, with mixer continuing to run at medium speed, slowly and steadily pour in the sugar. Continue to whisk until meringue is glossy and reaches medium-stiff peaks, about 8 minutes. Set aside.

Add one-quarter of the meringue to the chocolate mixture and fold until thoroughly combined. Add remaining meringue and gently fold until meringue is almost entirely incorporated.
Add one-quarter of the meringue to the chocolate mixture and fold until thoroughly combined. Add remaining meringue and gently fold until meringue is almost entirely incorporated.

Fill eight 6-ounce ramekins, individual cups, or a 2-quart baking dish with the mousse and refrigerate until the mousse has set, at least 4 hours and up to 5 days. Serve, topping mousse with dollops of whipped cream, if desired.
Fill eight 6-ounce ramekins, individual cups, or a 2-quart baking dish with the mousse and refrigerate until the mousse has set, at least 4 hours and up to 5 days. Serve, topping mousse with dollops of whipped cream, if desired.