Pre heat the oven at 180 deg C and line an 8 inch baking pan with foil.
In a bowl, add the flour, baking powder, bicarbonate of soda, ground cinnamon and apples. Mix well with a spatula and keep aside.
In another bowl, add the eggs, oil, yogurt and granulated sugar. Lightly whisk everything together.
Make a well in the centre of the flour mixture and pour in the egg mixture.
Mix together with a spatula until everything is just moistened. Do not overmix.
Spoon the batter into the lined baking pan and smooth the top.
Sprinkle the demerara sugar over the top.
Place the pan in the centre of the pre heated oven and bake for 30 minutes or till a toothpick inserted through the centre of the cake comes out clean.
Remove the cake from the oven and place it on a cooling rack.
After a few minutes, lift the cake from the pan and remove the foil.
Place the cake back on the rack and cool completely before storing in an air tight container.