In a 3-quart saucepan, bring 4 cups of water to a gentle simmer over medium heat. Place the chocolate and heavy cream in a stainless-steel mixing bowl that fits comfortably over the saucepan without touching the water. Set the bowl over the simmering water, stirring occasionally with a silicone spatula, until the chocolate melts completely. Stir the mixture until smooth and homogenous. Once blended, remove the bowl from the heat and place it on a heatproof surface.

Whisk the egg yolks into the chocolate mixture, ensuring they’re fully incorporated. The chocolate should feel warm but not hot to the touch. Set this mixture aside.

In the bowl of a stand mixer fitted with a whisk attachment, combine the egg whites, kosher salt, espresso powder, and cream of tartar. Begin whisking on medium speed until the egg whites start to foam. Gradually pour in the sugar, continuing to whisk at medium speed. Whisk until the meringue is glossy and forms medium-stiff peaks, which should take approximately 8 minutes.

Gently fold one-quarter of the meringue into the chocolate mixture, blending thoroughly to lighten the mixture. Carefully fold in the remaining meringue, gently folding until nearly all traces of meringue are incorporated without deflating the mixture.

Divide the mousse among eight 6-ounce ramekins, individual cups, or pour into a 2-quart baking dish. Place in the refrigerator to chill until set, at least 4 hours or up to 5 days. When ready to serve, garnish with a dollop of whipped cream, if desired.