Preheat oven to 350 and grease and flour 2 9" round cakepans

In a small saucepan bring the cream to a simmer over medium heat.

Pour over chopped chocolate into a large bowl, Allow to sit for 5 minutes to let chocolate soften, Stir until smooth, then add sour cream and vanilla, Stir well.

In another bowl whisk together flour, baking powder, baking soda and salt and set aside

In a stand mixer fitted with paddle attachment cream together 3/4 cup of the sugar and the butter for about two minutes, then switch to whisk attachment and beat for an additional 5 minutes until soft and creamy.

Beat in the egg yolks, one at a time.

Beat in chocolate cream mixture until well combined.

Transfer this mixture back into the large bowl and sift in the dry ingredients, folding with a rubber spatula until mixture is smooth with no lumps.

Meanwhile, clean the mixing bowl and whisk very well.

Add egg whites and a pinch of cream of tarter at medium/high speed for about 2 minutes.

Add remaining 1/4 cup of sugar and beat until stiff peaks form.

Fold 1/4 of egg white mixture into the chocolate batter to lighten it up.

Fold in the remaining egg white mixture until no streaks remain.

Transfer batter to prepared pans and bake for approximately 35 minutes or until skewer inserted in centre comes out clean..

Let cool on rack for 5 minutes before turning out of pans to complete cooling.

Ice with frosting of your choice.