Preheat the oven to 180ºc / 350ºf / Gas 4. Grease a 10 x 7 rectangle tin.
In a bowl, mix together the margarine and caster sugar until light and fluffy.
In a separate bowl, beat the eggs and add the vanilla extract. In another bowl add the flour and baking powder.
Sift a third of the flour in the butter mixture and combine, add the egg and stir together. Alternate until all combined. Pour into the tin and level.
Bake in the oven for 25mins, or until golden (see tip). Leave to cool.
Turn out onto a wire rack. Spread a layer of jam across the top and sprinkle with desiccated coconut and serve with custard.
