
Unsalted butter, for dishes
Granulated sugar, for dishes
¾ cup granulated sugar, divided
4 ounces bittersweet chocolate (67% cacao), finely chopped
⅓ cup unsweetened Dutch-process cocoa powder
⅓ cup boiling water
2 teaspoons pure vanilla extract
4 large egg yolks, slightly beaten
6 large egg whites, at room temperature
½ teaspoon cream of tartar
Powdered sugar, for garnish