Preheat the oven to 375 degrees F. Grease a 26 cm springform cake pan with butter and set aside. Sift the spelt flour, baking powder and salt together into a medium sized bowl. Set aside.
Grate the lemon and set the rind aside. Add the lemon juice into a large bowl and remove any seeds that may have fallen into the bowl. Peel the apples one at a time, cut each apple into quarters, cut out the core from each quarter and toss them all in the lemon juice. Take each quarter in the palm of your hand with the outside of the apple facing up. Using a small paring knife, slice each quarter multiple times length wise about half way through. The slices should be about ½ cm apart. Cutting the apples like this creates the fanned look when baked. Cut each sliced quarter in half lengthwise and toss again in the lemon juice. Set aside.
In the bowl of a stand mixer with the paddle attachment or in a large bowl with a hand mixer, beat the butter until smooth. Gradually add the sugar and beat again until lighter in colour and completely incorporated. Add the eggs one at a time and beat after each to combine. Add the vanilla extract and lemon rind and beat again until combined.
Add the flour mixture a little at a time on the lowest speed. When it is about half mixed together add the milk and mix on low until just incorporated. The dough should have a firm dropping consistency (should slowly but gradually fall off a spoon). If it does not add more milk 1 tbsp at a time until the correct consistency is achieved.
Spread the mixture evenly into the greased spring form pan. Lay the apples side by side in two ring formations with the outside of the apples facing up. Use a small paring knife to trim the apples for the inner ring if required.
Bake for 40 minutes turning the pan after 20 minutes to ensure even baking or until cake is golden brown and apples are soft. Remove from the oven and let the cake cool completely before removing from the spring form pan. Dust with icing sugar just before serving.