Preheat the oven to 180°C.
Line a 15cm deep round cake tin, or a 20x10x8cm loaf tin with baking paper, making sure that the paper extends a little above the top of the tin.
Cream the butter, sugar and zest until very pale and fluffy.
Beat in the eggs one at a time, until the mixture is smooth.
Fold the sifted flour and baking powder gently into the mixture with the milk, until just everything is combined.
Scoop the batter into the prepared cake tin, level the top and sprinkle with caster sugar.
Put the tin on a rack below the centre of the oven. Set the timer for 15 minutes, by which stage the top of the cake will have set slightly. Open the oven and carefully place the 2 strips of peel in the centre of the cake. Close the door gently and bake for another 30-35 minutes. Test with a fine skewer poked into the middle of the cake – it should come away clean.
Cool on a rack for 10 minutes, before turning out onto a rack. Store airtight.
Best eaten the day after you make it.