For the ramekins

1. Prep ramekins. Spray 8 (5-ounce) ramekins with cooking spray. If desired, pour sugar into one ramekin and tilt to coat the bottom and sides. Tap out excess sugar into the next ramekin and repeat until all the ramekins are coated. Set aside.

For the soufflés

1. Prep oven. Heat oven to 350 degrees F.
2. Make chocolate base. In a small saucepan, whisk together the powdered sugar, cocoa powder, and flour. Add the milk and water and whisk until smooth. Cook cocoa mixture over medium-low heat, whisking constantly, until thickened, 10-15 minutes. Transfer chocolate mixture to a medium bowl to cool. Set aside.
3. Whip eggs whites. In the bowl of an electric mixer fitted with the whisk attachment, beat the egg whites with the cream of tartar on medium-high speed until soft peaks form. Slowly add the granulated sugar and beat until stiff peaks form. Set aside.
4. Fold. Whisk the egg yolks and vanilla extract into the cocoa mixture. Fold about a quarter of the egg whites into the cocoa mixture to lighten it. Fold the remaining egg whites into the cocoa mixture just until combined. Divide mixture evenly among the prepared ramekins.
5. Bake. Bake the soufflés for 15 to 20 minutes or until the soufflés have puffed high above the ramekins and a toothpick inserted into the middle comes out with moist crumbs clinging to it.
6. Serve. Sprinkle with powdered sugar and serve immediately.