Preheat the oven to 375°F (190°C). Butter the inside of a soufflé dish and coat with granulated sugar, tapping out the excess.
In a saucepan, melt the chocolate and 2 tablespoons of butter over low heat, stirring until smooth. Remove from heat and set aside.
In a separate saucepan, whisk the flour into the milk over medium heat. Cook, stirring constantly, until thickened. Remove from heat and combine with the chocolate mixture.
Add the vanilla extract, egg yolks, and salt into the chocolate mixture, stirring until smooth.
In a mixing bowl, beat the egg whites with cream of tartar on medium speed until soft peaks form. Gradually add 2 tablespoons of sugar, beating on high speed until stiff peaks form.
Gently fold the egg whites into the chocolate mixture in three additions, being careful not to deflate the batter.
Spoon the mixture into the prepared soufflé dish, smoothing the top with a spatula. Run a thumb along the edge of the dish to create a ‘top hat’ effect.
Bake in the preheated oven for 12 to 15 minutes or until the soufflé has risen and the top is set. Serve immediately.