Place apples, sugar, water, lemon zest and cinnamon in a pot with a lid. Cook on a medium to low heat until the apples are soft. Use a whizz stick to puree them (or leave chunky if preferred). Set aside.
Grease and line a 20cm x 20cm square baking tray with baking paper and preheat oven to 180C (350F).
Place butter and sugar in a bowl of an electric mixer and cream until light and fluffy (about 4 minutes). Add eggs and vanilla and mix on low until just combined.
In a medium sized bowl, sift the flour, baking powder and salt. Whisk to combine then fold this into the butter-sugar mixture until combined. Tip dough out onto a floured surface. Roll into a ball then cut in half. Wrap the dough in gladwrap then place in fridge for 10 minutes.
Roll one half of the dough out between two pieces of baking paper (like I do in this recipe) into the size of the prepared baking tin. Peel the top layer of baking paper off and place the rolled out dough into the baking tin. Peel off the second piece of baking paper.
Spread the apple filling over the dough until the who surface is covered.
Repeat step 5 for second piece of dough. Place on top on the apple filling. Sprinkle sugar on top and bake for 20-25 minutes or until golden brown. Leave to cool in pan before lifting out and cutting into squares. Garnish with icing sugar if desired.