Combine all ingredients except soda water and garnish in a cocktail shaker with a handful of ice. Shake for 10-15 seconds, then strain out ice, and dry shake for another 15-20 seconds to really get the egg whites fluffy. (Be sure to hold the lid on tight as pressure will build inside the shaker from the egg whites!)
Strain into two tall serving glasses. Allow the fizz on top to settle for a minute, then gently pour the soda water into the glass until it's full (I poured mine over the back of a spoon to angle it to the edge of the glass about 1/4-1/2" below the rim). Garnish with cocoa powder before serving with a straw.
This cocktail was inspired by Holy City Handcraft's Salted Peanut Butter Fizz, but adjusted to make with the ingredients I had on hand.
If you don't have cognac on hand, just leave it out. I've tried it both ways and thought both were delicious!