Preheat your oven to 375°F (190°C). Generously grease four 6-ounce ramekins with butter, then dust with granulated sugar.

In a saucepan, melt the butter over medium heat. Stir in the flour until smooth, then gradually add the milk. Cook until thickened, then mix in chopped semi-sweet chocolate until melted and smooth. Add egg yolks, vanilla extract, and a pinch of salt, stirring until well combined.

In a separate bowl, beat egg whites with cream of tartar until soft peaks form. Slowly add sugar while whipping until stiff peaks are achieved.

Gently fold one-third of the whipped egg whites into the chocolate base, then fold in the remaining egg whites carefully.

Spoon the soufflé batter into prepared ramekins, smooth the tops, and run your thumb around the inside edges.

Place ramekins on a baking sheet and bake for 12-15 minutes until puffed and set.

Dust with powdered sugar and serve immediately.