Next, whisk egg whites until soft peaks form. I used a hand-held mixer because I don't have a stand mixer. Using electric mixers is very useful and time-saving. I think my arm would break off if I didn't have one. Add in some of the sugar and all the salt until medium firm peaks form.
It's not too bad. Be careful not to overmix which I always end up doing because it's so fast with the electric mixer.
Then gently fold the egg whites into the chocolate mixture using a spatula.
It doesn't have to be perfect, but try to not deflate the egg whites. And that's it! So I used a pot for melting, a bowl for egg whites, a bowl for egg yolk mixture, one beater (which I beat the egg whites first so I didn't have to wash and clean for the egg yolk mixture), and spatula. Nothing to it!
Line up your cupcake tin with liners. This is very important to have liners because the cupcakes will not come out even if you butter your tin generously. They're too delicate.
Fill them up about 3/4ths the way. I had a little more leftover from the 12, so I filled up the rest in a ramekin.
Bake and voila! First they're slightly puffy, but then they fall...hence the name fallen soufflé.
6 tablespoons (3/4 stick) (86 grams) unsalted butter, cut into pieces
Heaping 1/4 teaspoon (1 gram) espresso or instant coffee powder
3 large eggs, separated
6 tablespoons (97 grams) sugar, divided
1/4 teaspoon salt
1 teaspoon vanilla extract
Preheat oven to 350°F. Line 9 standard-size (3-ounce) muffin cups with paper liners. Stir chocolate, butter and espresso powder together in heavy medium saucepan over low heat mostly melted, then remove from the heat and whisk until it is fully melted and smooth. (I like to put the butter underneath the chocolate in the pan, so that it protects the chocolate from the direct heat.) Cool to lukewarm, stirring occasionally.
Using electric mixer (a hand mixer, rather than a stand mixer, actually works best here because the volumes are so small) beat egg yolks and 3 tablespoons sugar in medium bowl until mixture is very thick and pale, about 2 minutes. Briefly beat lukewarm chocolate mixture, then vanilla extract, into yolk mixture. Using clean dry beaters, beat egg whites in another medium bowl until soft peaks form. Gradually add remaining 3 tablespoons sugar and all of the salt, beating until medium-firm peaks form. Fold whites into chocolate mixture in 3 additions. Divide batter among prepared cups, filling each three-fourths of the way.
Bake cakes until tops are puffed and dry to the touch (some may crack, embrace it) and a tester inserted into the centers comes out with some moist crumbs attached, about 15 to 20 minutes. Cool in pan on a cooling rack, where the cupcakes will almost immediately start to fall.