First, make the topping. In a small pan heat the butter, sugar and rum (if using), on a low heat until dissolved, bubble for a few mins until slightly thickened. Divide between 6 really well-greased, 200ml ramekins. Add a pineapple ring with a glace cherry in the middle to each.
Heat oven to 180C/160C fan/gas 4. Beat together the butter and sugar with an electric whisk until pale and fluffy. Gradually beat in the eggs, then fold in the dry ingredients. Finally stir in the milk and divide the mixture between the ramekins. Put them on a baking tray and cook for 25-30 mins until risen, golden and springy to the touch.
To serve, turn the ramekins upsidedown onto a plate or a bowl and tap gently to release. Serve with custard.