Preheat your oven to 375°F. Grease four ramekins with butter and dust with granulated sugar, tapping out the excess.

In a medium saucepan over medium heat, melt the 3 tablespoons of butter. Stir in the flour and cook for about 1 minute, stirring constantly.

Gradually whisk in the milk, ensuring there are no lumps. Continue to cook, stirring frequently, until the mixture thickens, about 3-5 minutes. Remove from heat and stir in 1/4 cup of granulated sugar and the vanilla extract. Let it cool slightly.

In a large bowl, whisk the egg yolks until they are pale and slightly thickened. Gradually add the milk mixture to the egg yolks, whisking constantly to prevent curdling.

In another bowl, beat the egg whites with the cream of tartar and a pinch of salt until soft peaks form. Gradually add the remaining 1/4 cup of granulated sugar and continue to beat until stiff peaks form.

Gently fold one-third of the beaten egg whites into the yolk mixture to lighten it. Then, carefully fold in the remaining egg whites until just combined. Be cautious not to deflate the mixture.

Divide the soufflé mixture evenly among the prepared ramekins. Place them on a baking sheet and bake for 20-25 minutes, or until puffed and golden brown.

Serve immediately, dusted with powdered sugar.