Preheat your oven to 180C/160C Fan, and line two 8" cake tins with parchment paper.
Beat together your butter and sugar until light and fluffy.
Add in the flour, cocoa powder and eggs, and beat again until combined.
Split evenly between the two tins, and bake in the oven for 28-32 minutes. Sometimes it can take a little longer - don't open before 25 minutes, and be as quick as you can checking with a skewer.
Once baked, leave to cool in the tin for 10 minutes, and then transfer to a wire rack to cool fully.
For the Chocolate Buttercream
Make sure your butter is at room temperature. Don't use a spread such as margarine or stork.
Beat your butter on its own for about a minute, to loosen it up.
Add in the icing sugar gradually, beating in during or in 1/3 at a time. Don't add it all in at once.
Also, add in the cocoa powder, beating in fully.
If it's really very stiff, add in ONE tablespoon of boiling water at a time, beating fully each time, until it becomes smooth and lovely.
To Decorate
Place your first sponge on a plate/cake board, and pipe/spread on half of the buttercream frosting. Add on the second cake, and pipe/spread on the rest of the buttercream.
Add on anything you fancy as decoration, I used sprinkles. Enjoy!
Notes
This cake will last at room temperature (not in the fridge as it will go hard) for three days.
I use a cake tin, or a cake box to store it.
You can use dark chocolate in place of the cocoa powder if you wish - use 100g for the sponge, and 100g for the buttercream.
If your cake isn't baked fully, your oven might be at the wrong temperature, or it wasn't mixed correctly. Keep it in, till a skewer comes out clean, and it's springy to touch.