Line a rimmed baking sheet with parchment and spray with a non stick spray. Set aside.
Separate egg yolks and egg whites. Add a pinch of salt to egg whites and beat them with a wire beater until soft peaks are form, gradually add about ½ of the sugar. Beat until stiff peaks are formed, don’t over beat. Scrape out the meringue into a bowl. Into the same bowl you used for the meringue add egg yolks and remaining sugar, beat until thick and pale in color (ribbon stage), add vanilla extract and beat well.
In a separate bowl sift together flour, cocoa powder and espresso powder.
Into the egg white mixture fold in flour alternating with meringue.
Pour cake batter into a prepared baking sheet. Bake at 350 F for about 15 -18 minutes. Let cool completely before using.
Cut out round based on the size of the cup you are using. Use the left overs for cake pops or freeze for later.