1/3 cup plus 1 tablespoon bread flour or all-purpose flour
1 cup cold milk
6 large egg yolks
5 large egg whites
1 teaspoon pure vanilla extract
1/3 cup plus 1 tablespoon granulated sugar
1/4 cup powdered sugar
Preheat the oven to 375°F. Line a baking sheet with parchment paper.
To prepare the ramekins: Brush the bottoms and sides of eight 5-ounce ramekins with the melted butter. Divide the granulated sugar among the ramekins and turn them to thoroughly coat the bottoms and sides. Discard any extra sugar.
To make the soufflés: In a small stainless steel bowl or the top of a double boiler, combine the chopped chocolate and chocolate chips. Nest the bowl over a pot of barely simmering water (the bowl should not touch the water) and stir the chocolate until melted (taking care not to get any water in the bowl). Set the bowl aside off the heat.
In a medium saucepan, melt the butter over medium heat. Sprinkle in the flour and whisk until the flour is incorporated and the mixture thickens, about 1 minute. Reduce the heat to low and whisk in the milk. Continue whisking until the mixture becomes smooth, 2 to 3 minutes. Remove the saucepan from the heat. Transfer the mixture to the bowl with the melted chocolate and stir to combine. Slowly stir the egg yolks into the chocolate mixture. Set aside.
In a bowl, with an electric mixer fitted with the whisk, whip the egg whites and vanilla until the whites start to get foamy, then sprinkle in the sugar. Continue whipping the egg whites on medium speed until they form soft peaks and the consistency resembles whipped cream, about 3 minutes.
Use a spatula to fold about one-third of the egg whites into the chocolate mixture, carefully lifting from the bottom and folding over. Fold in half the remaining egg whites, then the last of the egg whites, taking care not to deflate the mixture.
Divide the mixture among the prepared ramekins and place them on the prepared baking sheet.
Bake the soufflés undisturbed until they have risen over the top of the rims, 12 to 15 minutes.
Pour the powdered sugar into a sifter or fine-mesh sieve and gently tap over each soufflé immediately as it comes out of the oven.