Combine the chocolate and butter in a small saucepan. Over very low heat, stir until melted*. Remove from heat and cool to room temperature. With a whisk, add the egg yolk, beating until thoroughly mixed.
In a large mixing bowl, whip cream just until it begins to thicken. Gradually add the sugar, and continue beating until the mixture is stiff. Stir a small amount of the whipped cream into the melted chocolate to lighten it. Fold in the remaining cream and place in four individual souffle dishes (6 oz. capacity). Chill at least 30 minutes before serving. Top with shaved chocolate or dust with cocoa. 4 servings.
*This step can also be completed in the microwave at the lowest setting.