Meanwhile, place the chocolate, butter and coffee in a large, heat-proof bowl set over a saucepan of barely simmering water. Stir occasionally until completely melted and smooth and very warm (the mixture should register 115 degrees F on an instant-read thermometer). Remove from the heat and stir in the salt and vanilla extract.
About 30 minutes before serving, run a thin knife between the cake and sides of the pan; remove the sides of the pan. Place the cake on a serving platter, dust with powdered sugar and serve with a dollop of fresh whipped cream, if desired.
Nutritional values are based on one serving