Preheat the oven to 180 C/350 F.
In a medium mixing bowl, using your hands, mix together the flours, butter (cube it first - smaller will be easier), spices, salt, baking soda and cornflour until it forms a breadcrumb like texture.
Add the sugar (brown sugar) and mix together again.
Using a hand mixer or a standing mixer, mix together the above ingredients you have already combined with your hands that resemble breadcrumbs, and now add the egg, vanilla and gradually add the milk. You may not need all of it. You want to mix slowly until a thick batter forms.
Prepare the apples - cutting them into cubes (peeled and cored), small bite size. Do not grate the apples. You want to see them. You want to taste them, which is why you want to choose an apple you enjoy eating as you will taste its flavor, albeit cooked and thus softened.
Now add the apples to the batter and mix with a wood mixing spoon until evenly distributed throughout the batter.
Prepare the cake pan you prefer. I used my French 3" x 5" rectangular cake pan and still had enough batter left over for a small single cake if I wanted to make it. There is enough batter for a full cake - 7-8" in diameter.Whichever you choose, eliminate any headaches of trying to remove the cake from the pan and line it with parchment as you see in the photo. Put the batter into the baking pan, and fill about 2/3 - 3/4 of the way up the sides of the pan.
Now sprinkle as much or as little of the Demerara sugar that you want on top. I like to cover the entire top of the cake as it gives a nice crunchy topping that provides a different and complementary texture to the cake and apples.
Bake for 40-50 minutes. Checking the center to ensure cooked through, but don't overbake as it will dry out the apples. For a round cake pan, you may only need to cook it 30-40 minutes as the surface areas is larger and the filling thinner. So check often as you hit the 30 minute mark. If the top begins to get too brown, but the cake itself is not baked through yet, simply cover the top with tin foil for the remainder of the baking period. Remove when just cooked through. Let cool for 10 minutes and then remove to place on your serving platter.