Preheat the oven to 170 C / Gas 3. Grease 2 cake tins.
Put butter or margarine and caster sugar in a bowl and mix together until light and fluffy.
Add eggs one at a time, making sure you mix in well each time.
Sift the flour and baking powder together and put half into the mix. Mix well. Add the rest of the flour. Mix again, and then add the cocoa powder. Add the vanilla extract and mix again well. Divide the mix between the 2 greased cake tins.
Bake in oven until a sharp knife inserted near the centre comes out clean, about 55 minutes.
Once cooled, spread one side with the raspberry jam.
Whisk the whipping cream until very thick and then spoon it over the raspberry jam, place top half of cake on top of cream, pressing in place firmly.
Give a light dusting of icing sugar over the top of the cake.