For filling 2/3 cup sugar 1/4 cup cornstarch 1/2 teaspoon salt 4 large egg yolks 3 cups whole milk 7 oz fine-quality bittersweet chocolate (66% Vahlrona, 71% Organic & fairly traded very dark chocolate), melted 2 tablespoons unsalted butter, softened 1 teaspoon vanilla Whisk together sugar, cornst