Grease 2–3 ramekins with 1 tablespoon unsalted butter, then lightly dust with sugar. This prevents sticking and helps the soufflé rise evenly.
Separate the yolks from the whites into two clean bowls. Make sure no yolk gets in the whites.
In the yolk bowl, whisk together 3 tablespoons granulated sugar, ¼ cup whole milk, 1 teaspoon vanilla extract, and 2 tablespoons all-purpose flour until smooth and lump-free.
Using a hand mixer, beat the egg whites until stiff peaks form.
Gently fold the whipped egg whites into the yolk mixture in batches. Use slow, careful motions to preserve the air.
Spoon the batter into prepared ramekins, filling about ¾ full.
Place ramekins in the air fryer basket and cook at 320°F for 10–12 minutes, until puffed and set.
Dust with 1 tablespoon powdered sugar and serve right away. Soufflés deflate quickly once removed from heat.