Prepare the ramekins by brushing the inside with butter and coat with sugar. Set aside. Preheat oven to 390 degrees F.
Over medium heat, heat up the butter until it melts. Add the flour and whisk vigorously until the flour is incorporated.
Add the hot milk and keep on whisking until it becomes a paste consistency. Take off heat.
Add the vanilla extract and whisk it again. Add the egg yolk one by one and mix again. Transfer into a glass bowl so it can cool off. This is the roux mixture.
In a stand mixer, whisk the egg whites until it becomes foamy. Add the cream of tartar and whip until it becomes soft peak.
Add the 4 tbsp of sugar, one tablespoon at a time, while the machine is whipping up the egg whites. Keep on whisking until it becomes stiff peak. This is the meringue.
Add 1/3 the meringue into the roux mixture. Whisk until all is combined. Add the rest of the meringue and gently fold the egg whites with a spatula.
Spoon the souffle into the ramekins to the top of the ramekins. Run your thumb around the inside of the ramekins. Put the ramekins onto a baking sheet and bake for 12-15 minutes.