Using electric beaters, beat butter ½ cup caster sugar and vanilla in a medium bowl until sugar dissolves and mixture is pale and creamy. Add eggs one at a time. Beat well after each addition.
Using a spoon remove pulp from custard apples. Discard seeds and add pulp to mixture. Mix well until combined.
Using a large metal spoon, fold flour in 2 batches into mixture. Spoon into loaf pan and smooth the top.
Combine extra sugar and cinnamon and sprinkle over mixture. Bake for 1 hour or until a skewer comes out clean. Remove from oven and stand for 5 minutes before turning cake out onto a wire rack to cool.
Slice and serve.