In a double boiler or small saucepan set over simmering water, combine the chocolate and butter. Heat, stirring often, just until melted and smooth. Remove from the heat.
Combine the egg yolks with half of the sugar in large bowl of an electric mixer. Beat for several minutes, until the yolks are thick and pale yellow.
In another large bowl (with clean beaters), beat the whites with the remaining sugar until soft peaks form.
Pour the chocolate mixture into the bowl containing the egg yolks and stir with a rubber spatula to blend. Fold the egg whites into chocolate mixture gently, but thoroughly. Transfer the mousse to a serving bowl; refrigerate, covered, for 12 hours. When ready to serve the mousse, dust with cocoa powder or top with chocolate curls, if desired.
100-gram (3.5 ounce) bars are standard weight for imported chocolate.
Recipe courtesy Van Gogh's Table at the Auberge Ravoux (Artisan, 2001) by Alexandra Leaf and Fred Leeman