Melt the chocolate and butter in the microwave or in a medium metal bowl
set in a skillet
of barely simmering water, stirring with a rubber spatula until smooth. Remove the bowl from the water bath and set aside to cool slightly. In the bowl of a stand mixer fitted with the whisk attachment, combine the eggs, sugar, vanilla, salt, and 2 tablespoons water. Beat on medium high speed until the mixture is very foamy, pale in color, and doubled in volume, 2 minutes. Reduce the mixer speed to low and gradually pour in the chocolate mixture. Increase the speed to medium high and continue beating until well blended, about 30 seconds. Add the cocoa powder and mix on medium low just until blended, about 30 seconds.