3 tablespoons unsalted butter 6 ounces semisweet chocolate, best quality 3 large eggs, at room temperature, yolks and whites separated ½ teaspoon cream of tartar ¼ cup plus 2 tablespoons sugar ½ cup heavy cream, cold ½ teaspoon vanilla extract FOR SERVING Place the butter in a medium microwave-safe bowl. Break the chocolate into small pieces directly into the bowl. Microwave it…