Preheat oven to 350 F (180 C). Line an 8 X 8-inch square baking pan with parchment paper (see parchment paper trick at bottom of post).
In a large glass bowl, over a double boiler, melt: bittersweet chocolate, cream and butter. Remove bowl from heat and let stand for about 10 minutes.
Stir in vanilla, sugars and eggs until well combined. Using a rubber spatula stir in dry ingredients until just combined.
Place batter (it will be thick) into prepared pan. Bake in center of preheated oven for about 30 minutes until edges are set but centre is slightly jiggly.
Remove from the oven and cool completely while making frosting.