Preheat the oven to 325°F.
Grease an 8 inch cake pan and line the bottom with parchment. Set aside.
In a saucepan set over low heat, or in a microwave, melt together the chocolate and butter, stirring till smooth.
Beat in the egg yolks, one at a time, then add the flour, sugar and hot water, stirring till smooth.
In a separate bowl, beat the egg whites till stiff but not dry. Fold them into the chocolate mixture.
Spoon the batter into the prepared pan and bake for 25 minutes (cake won't look done in the center).
Remove the cake from the oven, leaving it in the cake pan, and cool completely on a wire rack. Refrigerate until ready to serve.
Remove the cake from the pan, place it on a plate and serve topped with sweetened whipped cream.