Place a sheet of parchment into a 6 inch round pan and press against the bottom and sides. Or you can butter and flour a 6 inch round springform or loose bottomed pan.
Melt the butter and chopped chocolate together in a double boiler or microwave.
Separate your eggs .
In a bowl, beat the egg whites until frothy.
Add 1/4 cup of sugar , little by little while beating . Beat until you get firm white peaks. Will take 5-7 minutes.
In another bowl , beat the yolks with remaining 1/4 cup sugar until thick and pale. It will take around 3-5 minutes .
Mix in the coffee and salt . Beat for a minute.
Add the vanilla and butter-chocolate mixture and beat for a minute.
Fold in the whipped egg whites in three additions into the yolk mixture.
This batter is soft , light and like a mousse. Spread into your prepared pan and bake at 180 C for 30 -35 minutes or until a toothpick inserted in the centre comes out with few moist crumbs.
Cool completely and remove from pan. It'll fall once out of the oven and crack. Dust with cocoa powder if you wish.Or serve alongside ice cream or whipped cream . Or drizzle with a glaze.