Prepare ramekins: Butter the ramekins thoroughly using your fingers, making sure to cover the sides and edges. Surround each ramekin with a double-thick collar of buttered aluminum foil extending 3 inches above the rim to support the soufflé as it rises.
Melt chocolate: Melt the semi-sweet baking chocolate in a double boiler until smooth and silky. Keep warm on a back burner.
Make flour and milk base: In a saucepan, add the all-purpose flour. Slowly whisk in the milk constantly to avoid lumps, creating a smooth cream. Add butter and bring to a boil, stirring continuously for 2 minutes until thickened. Remove from heat and let cool for about a minute while stirring.
Add egg yolks: Add the egg yolks one at a time to the cooled flour-milk base, mixing well after each addition.
Combine chocolate and vanilla: Add the melted chocolate and vanilla extract to the base mixture and stir to combine.
Whip egg whites: In a mixer, beat six egg whites with a pinch of salt until soft peaks form. Gradually add sugar while continuing to beat until stiff peaks form.
Fold mixtures: Gently fold the chocolate base mixture into the whipped egg whites until just combined, being careful not to deflate the meringue.
Fill ramekins and bake: Ladle the soufflé mixture into the prepared ramekins, filling them as full as desired. Place ramekins in the oven and bake for 40 to 50 minutes until risen and slightly cracked on top.
Serve: Optionally dust with powdered sugar and serve immediately while still hot and creamy inside.