Pre-heat oven at 180o C.
In a bowl place the flour with the baking powder and leave to one side.
Cream together in the mixer the Margarine, Butter and Sugar.
Add one egg and mix for about 3 minutes then add the remaining egg mixing for another 3 minutes.
Turn the mixer to a lower speed and add the vanilla essence and the yogurt.
Finally slowly add the flour and baking powder and mix well until your mixture is nice and smooth.
Pour the cake mixture in a lightly buttered rectangular baking tray and bake at 180oC for approximately 45 minutes.