Chocolate Orange Gateaux

This one, I've been waiting for all week! because the big one is for an order, Ive made a mini duplicate one, so I could cut into it, and eat some, of course!
The ingredients listed are for one 8" deep cake.

For the cake:

9oz SR Flour
1tsp Baking powder
2tbsp Cocoa Powder
8oz Margarine
8oz Castor Sugar
4 Eggs
2tbsp Milk

For the Frosting:

8oz Dark Chocolate
8oz Sour Cream

8oz Double Cream
4oz Mascarpone
1tbsp Icing Sugar
orange food colouring
Chocolate Orange Flavouring
Mini Chocolate Orange Segments

Make the cake:

Cream the margarine and sugar together until light and fluffy.
Add beaten eggs and milk a little at a time, adding a little flour if needed, to prevent curdling. (scrape down bowl).
Sift together flour, baking powder and cocoa, and fold into cake mixture.

Place cake mix into greased and lined 8" round cake tin, and bake on Gas 5, until risen, and centre springs back, also use skewer to test if middle is cooked.

When removed from oven, leave in tin for about 20mins before removing, and cooling on wire rack.

To make Chocolate Frosting:

Melt chocolate in bowl of pan of simmering water, remove bowl from heat, and beat in sour cream for 5 minutes, leave to cool (it will thicken),

Orange Frosting:

Whisk together double cream, mascarpone, icing sugar and 2tsp of flavouring, add enough orange food colouring till colour required.

To decorate:

When cake is cool, with large serrated knife cut into 3 slices horizontally.
In the inside layers spread orange frosting first, then chocolate ganache.
Cover cake completely with chocolate ganache and smooth sides with palette knife. and fluff up top.

Pipe rosettes on top and side as required, and place mini orange segments to finish.

Enjoy!