Preheat oven to 350 degrees.
Line the bottom of a 9 inch cake pan with parchment paper.

Melt the chocolate & butter in a double boiler.  When the two are mostly melted, remove from the heat and whisk.  Cool slightly.  Whisk in 1/2 c sugar, egg yolks, then flour & salt.

Beat egg whites at medium speed until foamy.  Add a third of the 1/4 c sugar and beat until whites are opaque, then add another third.  When the whites increase in volume, add the last of the sugar and turn mixer to high and beat until soft peaks form.  Do not overbeat.

Fold one third of the whites into the chocolate mixture, then fold in the rest.  Pour into the pan and bake until a skewer comes out clean with a few crumbs clinging to it.  35 - 40 min.

Cool on a rack.  Unmold on a serving plate.  Remove the parchment paper and decorate by dusting with the cocoa powder and powdered sugar.