Chocolate souffle cupcakes

step 1
Preheat oven to 350 F. Line 9 standard-size muffin cups with paper liners. Stir chocolate, butter and espresso powder (if using) together in heavy medium saucepan over low heat mostly melted, then remove from the heat and whisk until it is fully melted and smooth. A piece of advice: put the butter underneath the chocolate in the pan, so that it protects the chocolate from the direct heat. Cool to lukewarm, stirring occasionally.

step 2
Using electric mixer (we used a hand mixer), 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.

step 3
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. It will be all the better to put your frosting on them.