Preheat oven to 180c/160c Fan/Gas Mark 4.
Line the bottom of two (20cm) sandwich/cake tins with greaseproof paper or oil them.

In a large bowl, whisk together butter and sugar until light and fluffy.
Gradually whisk in the eggs.
Gradually whisk in the baking powder, flour and the cocoa powder with a spoon and then give it a quick whisk.

Divide the mixture evenly between the tins (first time I made this I transferred the mixture into a measuring jug and scooped half the measurement into each tin).
Level and smooth the mixture in each tin with a palette knife.
Bake for 25-30 mins until firm to the touch.
Set aside to cool for 5 mins.
Run a palette knife around the inside edge of the tins and carefully turn the cakes out onto a cooling rack.

MOUSSE:
Place the chocolate in a bowl and melt over a pan of gently simmering water (do not allow the base of the bowl to touch the water).
Stir continuously, taking care not to let the chocolate get too hot or split.
Whip the double cream, carefully fold in the melted chocolate until smooth and not streaky.
To learn how to make whipped cream using double cream, check out my double cream recipe.

When the cake is cool (approx 1-2 hours), you can put your desired filling (cream, mousse, jam, etc) on the flat side of of the cakes.
Put the two sides of the cake together and decorate on top as desired.
I personally either:
Put mousse in the middle and on top
or
Put jam and cream in the middle and mousse on top.
You can level the mousse on top with a palette knife.
Chocolate orange mousse is good.

Fan oven tip.