- In a small saucepan, melt the butter over low heat. Remove from heat and add the chocolate. If the butter doesn’t fully melt the chocolate, return the pan to low heat for 10 seconds and stir until melted. Set aside to cool.
- Sift the flour, cocoa powder, and espresso powder into a small mixing bowl. Set aside.
- In a medium mixing bowl, combine the eggs, sugars, salt, and vanilla. Using a mixer with a whisk attachment (or hand mixer), beat on high speed until the mixture thickens, turns pale, and falls from the beater in wide ribbons that slowly dissolve on the surface, about 4-5 minutes.
- Fold the cooled chocolate mixture into the egg mixture using a rubber spatula.
- Add the flour mixture and fold in quickly but gently to avoid deflating the eggs.