Arrange the apple slices on top of this and sprinkle with the cinnamon and half of the orange zest.
Cream 175g/6oz butter, 175g/6oz soft brown sugar in a bowl and add the eggs gradually. Then stir in the flour and baking powder, orange juice of half the orange and remaining orange zest. Spread this mixture over the apples and bake for around 45 -50 minutes.
Turn the cake out onto a cooling rack and allow to cool. Melt the remaining butter and soft brown sugar in a pan, and whisk in the remaining orange juice. Pour the mixture over the top of the apples and allow to cool.