In a large bowl, beat together the softened butter and sugar just until creamy, for about 1 minute.
Beat in the eggs one at a time until well combined.
Stir in the vanilla extract and lemon zest.
In a separate medium sized bowl, whisk together the flour, baking powder, cinnamon and salt.
With the mixer on low, add the flour mixture to the butter mixture alternately with the almond milk making 3 additions of dry and 2 of wet.
Fold in the 1 cup of reserved diced apples.
Spoon the batter into the prepared pan and level the top with the back of a spoon or an off set spatula.
Layer the apple slices over the cake batter in a circular motion while slightly overlapping them.
Top with a generous sprinkle of demerara sugar.
Bake in preheated oven for about 45 minutes or until a toothpick inserted in the middle comes out clean.
Run a knife around the edge and let cool in the pan for bout 15 minutes.
Transfer to a wire rack to cool completely.
Dust with powdered sugar just before serving.
Enjoy with your favourite cup of coffee or tea.