Preheat oven to 170`c and grease and line a 23 x 13 cm loaf tin. Next, finely grate the rind of 1 1/2 lemons. Peel the rind off the remaining ½ lemon and put aside. Place the butter or margerine in a mixing bowl with the sugar and beat for 5 minutes until light and fluffy. Stir in the lemon rind and the vanilla essence, before adding in the eggs 1 at a time. It is important that you also add a tablespoon of the flour with each egg, beating for 1 minute after each addition. This will stop the mixture from becoming to loose and curdling. Next, sift over the remaining flour with the baking powder and stir until just combined. Pour the cake mixture into the tin and level the top using a palette knife. Place the piece of lemon rind on the top of the cake and bake in the oven for 1hour to 1hour 15 minutes, or until a metal skewer inserted in the middle comes out clean. When ready, remove the Madeira cake out of the oven, and allow to cool in the tin on a wire rack for 15 minutes, before turning out of the tin to cool completely