Steer clear of Ganache filling it's a nightmare and too overpowering.
I have a secret weapon for chocolate cake. It's so quick and simple. For a simple recipe:
I make a basic sponge with your 225g butter, 225g sugar, 225g SR flour, 4 eggs this is your base for any sponge cake. Add liquid so eg 50g of yoghurt, buttermilk, or 2-3 tablespoons of milk, this keeps it moist.
Follow a normal sponge cake recipe for 2x 20cm round cake tins, but add the chocolate like this:
Mix cake in the usual way,
Use 50g buttermilk and 1tsp strong vanilla essence for your liquid.
Mix 2 tbs of cocoa powder with 2-3tbsp of boiling water and add that in addition to your liquid after the egg mixing stage.
Put mix into 2 x 20cm round tins.
Then after it's out of the oven you poke some holes in both cakes while warm (not scorching hot though, leave for a minute) and pour chocolate syrup over the cake.
Imagine making a lemon drizzle cake, but actually you're making a chocolate drizzle cake.
Making chocolate syrup is really basic. Just mix coco with sugar and hot water. Pour it over the warm cake.
Make a filling of your choice and squish it in between the two cake layers!