Preheat the oven to 180°C (350°F/Gas 4). Grease a 20cm (8inch) round shallow cake tin and line the base with baking paper.
Cream the butter and sugar in a small bowl using electric beaters until light and fluffy. Add the egg gradually, beating thoroughly after each addition. Beat in the vanilla, then transfer to a large bowl. Using a metal spoon, fold in the sifted flours alternately with the milk. Stir until the mixture is smooth.
Spoon into the prepared cake tin and bake for 30 minutes, or until a skewer inserted into the centre of the cake comes out clean. Leave in the tin for 5 minutes, then turn out onto a wire rack.
To make the topping, brush the warm cake with the butter and sprinkle with the combined sugar and cinnamon.