Preheat your oven to 375°F (190°C). Grease the ramekins with butter and dust with sugar. Set aside.
In a small saucepan, whisk flour and milk over medium heat until thickened. Remove from heat and let it cool slightly.
Whisk in the egg yolks and vanilla extract until smooth.
In a clean bowl, beat egg whites and cream of tartar until foamy. Gradually add sugar and beat until stiff peaks form.
Gently fold the egg white mixture into the yolk mixture. Don’t overmix—you want to keep as much air as possible.
Spoon the mixture into prepared ramekins and run your thumb around the inside edge to help them rise evenly.
Place ramekins on a baking sheet and bake for 13–15 minutes, or until puffed and golden.
Dust with powdered sugar and serve immediately. They’ll start to fall after a few minutes—but they’ll still taste like a dream.