Preparation Melt chocolate and butter in a large metal bowl set over a saucepan of barely simmering water, gently stirring occasionally until smooth. Remove from heat. Meanwhile, beat yolks in a small bowl with an electric mixer until thick enough to form a ribbon that takes a few seconds to dissolve, 2 to 4 minutes. Whisk yolks into chocolate mixture along with Cognac, then cool to warm. Beat cream in a medium bowl with cleaned beaters until it just holds stiff peaks. Beat whites with salt in another bowl with cleaned beaters until they just hold soft peaks. Fold whipped cream and beaten whites into chocolate mixture gently but thoroughly. Transfer to stemmed glasses, 4-ounces ramekins, or a serving dish. Cooks' Notes:•The eggs in this recipe are not cooked.•Mousse can be chilled, its surface covered with parchment paper, up to 2 days. Let stand at room temperature at least 30 minutes before serving.