Italian Flourless Chocolate Cake

tablespoons/100 grams unsalted butter softened, plus more for pan

cup/60 grams confectioners’ sugar, sifted, plus more for dusting

ounces/113 grams 70 percent “top quality” dark chocolate

extra-large eggs, separated

tablespoons/20 grams potato starch

tablespoons/40 grams superfine sugar