Melt butter in a sauce pot over medium heat, whisk in flour, salt and then milk and cook until it comes to a boil for a few seconds and thickens. What you have just made is a very basic béchamel sauce.
Off the heat, stir in chocolate and stir constantly until it's melted.
Add coffee, vanilla, and ¼ cup of sugar, whisking until smooth.
One at a time, add the egg yolks while whisking constantly. With the addition of these ingredients, you have just made chocolate pastry cream, or chocolate pudding.
Cool, cover and chill the base for at least four hours and up to a day ahead.
About an hour before you want to serve the soufflé
Preheat oven to 375ºF.
Coat the inside of the baking dish with butter and sugar.
Butter a 2-2½ quart soufflé dish or straight-sided baking dish. Add about 2-3 tablespoons of granulated sugar, and then tap and rotate the dish to cover the bottom and sides with sugar.
In a perfectly clean and dry bowl, whisk the egg whites to soft peaks, and gradually whisk in the remaining ¼ cup of sugar. Continue to whisk until the peaks are stiff and glossy.
Stir ¼ of the whites into the soufflé base to lighten it. Then, thoroughly but gently fold in the rest of the whites.
Fill the soufflé dish all the way to the top. If you’d like, run your thumb around the top inside of the soufflé dish to clean off the rim. Some chefs say this helps the soufflé to rise more evenly.
Bake for 35 - 40 minutes until the soufflé rises 2 inches above soufflé dish and/or the top of the soufflé is golden brown.
Dust with confectioners sugar and serve immediately because the soufflé will deflate in less than two minutes.
For individual soufflés, butter and sugar individual ramekins and bake for about 20 minutes, or until well risen.