• 4 ounces semisweet or bittersweet chocolate , coarsely chopped
• 4 tablespoons unsalted butter , sliced
• 4 large eggs , separated
• 1/8 teaspoon cream of tartar

In a heavy saucepan over low heat, heat chocolate and butter, stirring occasionally, until melted. Remove chocolate mixture from heat and whisk in egg yolks, one at a time, until blended. Transfer to a large bowl and set aside.

In the bowl, beat egg whites with an electric mixer at medium speed until foamy. Add cream of tartar; beat until stiff peaks form.

With a rubber spatula, gently fold one-third of beaten whites into chocolate mixture. Fold in remaining whites until mixture is just smooth and blended.

Pour mousse into a serving bowl or individual glasses. Cover and refrigerate until firm, at least 4 hours or overnight.