In a bowl, beat the heavy cream until it forms stiff peaks, cover and set in the fridge while you work on next steps in this recipe.
In a small bowl, whisk the egg yolk and sugar for 2 to 3 minutes until it's pale in color and the sugar dissolves, set it aside.
In a small saucepan, add about 1/2 inch of water and bring it to a simmer on medium heat. Place a small mixing bowl over the saucepan and add the chocolate, water, espresso and butter and over medium to low heat, cook everything together just until the chocolate and other ingridents melt.
Spoon 1 tbsp of the chocolate into the egg yolk mixture and mix it just enough to combine. Pour all the yolk mixture into your chocolate mixture and mix it well then put it mixing bowl on the simmering water on the saucepan; double boiling it again.
Add the vanilla extract and stirring constantly, cook over low heat for a minute or until the mixture thickens. Let it cool for a few minutes stirring frequently.
In a small bowl, whisk the egg white until it forms stiff peaks. Fold the egg white in the cooled chocolate mixture.
Fold the chocolate mixture into the whipped heavy cream being careful not to over mix otherwise the heavy cream will lose its texture.
Pour the mixture into ramekins and refrigerate for a minimum of 4 to 6 hours to overnight.