Preheat the oven to 180°c / gas 4. Grease 3 8" cake tins and set aside.
Beat together the margarine and caster sugar with an electric whisk until pale and fluffy.
In a separate bowl, lightly beat the eggs and vanilla extract, in an other large bowl add the flour, baking powder and cocoa powder.
Add a ¼ of the eggs to the butter and with a wooden spoon stir in, then fold in ¼ flour and gently stir until combined, repeat until all the eggs and flour are mixed together.
Divide the mixture between the 3 tins. Bake in the oven for 20mins until you press in the middle and they spring back.
Leave to cool for 10mins, turn out onto a wire rack and leave to cool completely.