Heat the oven to 180C. Butter a cake tin and/or line with parchment paper.
Cream the butter and sugar together. Add the beaten eggs and vanilla and beat together. Slowly add in the flour (be as gentle and quick as possible as the flour tends to weigh down the beautiful lightness of the creamed butter and sugar).
Pour the batter into the prepared cake tin. Top with the apple wedges - sinking them in a little as you place them. Here''s where you''ll think oh-my-goodness there are too many apple wedges and not enough space, but they''ll shrink and you''ll be glad you added all the apples!
Sprinkle with the Demerara sugar and the cinnamon. Bake for about an hour or until the top is slightly browned and a skewer inserted in the center comes out clean. Leave to cool on a wire rack.
Serve warm with vanilla ice cream (the vanilla ice cream is not optional!).