Put the sugar and the butter into a bowl and mix with a wooden spoon or an electronic mixer.
Next add the flour and mix again.
Then crack the eggs into a jug and whisk until orange.
After that keep mixing, adding the egg a bit at a time, then add the cocoa powder, if using. Pour the cake mixture into 2 baking tins or 1 deep baking tin.
Bake in the preheated oven until brown and a skewer inserted near the centre comes out clean, about 20 minutes.
Then take the cake out of the oven, give it 30 to 60 minutes to cool down and add the cream and the jam.