Preheat your oven to 375°F (190°C). Grease four ramekins (6-ounce size) with butter and dust with granulated sugar, tapping out the excess.

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

Gradually whisk in the milk, continuing to stir until the mixture thickens and comes to a gentle boil. Remove from heat and stir in the vanilla extract.

In a large bowl, beat the egg yolks with 1/4 cup of granulated sugar until pale and creamy. Slowly add the milk mixture to the egg yolks, whisking constantly to prevent curdling.

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

Gently fold one-third of the egg whites into the yolk mixture to lighten it. Then carefully fold in the remaining egg whites until no white streaks remain.

Spoon the soufflé mixture into the prepared ramekins, filling them about three-quarters full. Run your finger around the edge of each ramekin to help the soufflés rise evenly.

Place the ramekins on a baking sheet and bake for 18-20 minutes, until puffed and golden. Do not open the oven door during baking.

Serve immediately, dusted with powdered sugar.