Preheat your oven to 350°F (180°C).
Grease cake tin thoroughly.
Line with parchment paper.
Peel apples carefully.
Chop into small pieces.
Set aside for later.
Cream butter and sugar together.
Beat until light and fluffy.
Add eggs one at a time.
Mix vanilla extract into mixture.
Sift flour and cinnamon together.
Fold flour into butter mixture.
Add milk gradually while mixing.
Fold chopped apples into batter.
Transfer mixture to prepared tin.
Sprinkle demerara sugar on top.
Bake for 45-50 minutes.
Test with skewer for doneness.
Cool in tin for 10 minutes.
Turn out onto wire rack.
Cool completely before serving.