Preheat oven to 350F. Butter the sides and bottom of a 9" springform pan and dust with cocoa powder.
Place chocolate and butter into a large heatproof bowl. Place bowl over a simmering pot of water to create a double boiler. Melt until smooth and combined.* Cool slightly and whisk in egg yolks and espresso powder.
While chocolate is cooling, start whipping the egg whites in a grease free bowl. Just before they reach soft peaks, start adding the sugar 1 Tbsp at a time. Beat until almost at stiff peaks and glossy.
Fold 1/4 of the whites into the chocolate mixture to lighten it. Fold in the remaining whites.**
Spread evenly into prepared pan and smooth the top.
Bak for approximately 35mins or until cake is set in the center (a toothpick won't be completely clean)
Cool completely in pan on a wire rack.
Cake should pull away from sides of the pan as it cools, but run a knife around the edges if needed and remove the sides of the pan.
Dust with powdered sugar or cocoa powder if desired. Or serve with whipped cream, ice cream, or caramel or raspberry sauce.***
Notes
* You can also melt it in the microwave. ** It's ok if there are some streaks of whites. *** Cake is easiest to cut when it is chilled. Cut slices about an hour before serving.