Heat oven to 220C.
Grease and flour a 26 cm round baking pan.
Using electric beater, beat egg yolks with 3/4 cup sugar and vanilla, until mixture becomes thick.
Add cocoa and flour and mix well.
Add butter and mix together, until batter is smooth.
In another bowl, beat egg whites gently with 3/4 cup sugar, until smooth and creamy.
Add batter and mix together gently and slowly, using a wooden spoon.
Pour mixture into baking pan and bake for 20 mins.
Reduce heat to 170C and bake for another 10 - 15 mins or until a knife comes out clean, when inserted in the middle of cake.
Remove from oven and leave to cool for 30 mins.
In a small saucepan, heat and stir chocolate over very low heat, until it melts.
Remove from heat.
Add cream and 2 cups of powdered sugar and stir, until sugar dissolves and mixture is smooth.
When cake is cold, cut it crosswise into two parts and stuff with jam and cover totally with chocolate sauce.
Refrigerate for 30 mins, before serving.