Special equipment: whipped cream charger and nitrous oxide cartridge
Place the eggs, sugar and salt into the bowl of a stand mixer with the whisk attachment and whisk for 1 1/2 minutes on medium-high speed. Add the melted chocolate, flour, espresso powder and vanilla and whisk on medium speed for 30 seconds.
Pour 2 cups into a whipped cream charger. Fill only halfway. Charge with 1 nitrous oxide cartridge. Shake 10 to 15 times. Dispense the foam into coffee mugs until one-third full.
Place 1 mug at a time in a microwave. Cook for 45 seconds at 90-percent power. Repeat with the remaining batter. Serve the cakes immediately.