Pre -heat oven to 180°C/160°C fan-forced. Grease a 4.5cm deep, 20cm round cake pan. Line base and side with baking paper.
Using an electric mixer, beat butter, sugar, egg and vanilla together for 2 minutes or until combined. Stir in half the flour, then half the milk. Repeat with remaining flour and milk. Spoon mixture into prepared pan. Level top.
Bake for 25 to 30 minutes or until a skewer inserted in the centre comes out clean. Stand in pan for 5 minutes. Turn, top-side up, onto a wire rack.
Make topping Combine cinnamon and sugar in a bowl. Brush hot cake with butter. Sprinkle with cinnamon mixture. Cool. Serve.