Heat your oven to approx Gas Mark 6 and line a smallish deepish roasting tin with parchment.
Mix all the cake ingredients (egg, flour, baking powder, granulated sugar and milk) together and pour into your lined tin. Melt your butter.
Sift over the cake surface all the demerara sugar, add a bit more if you like it extra gooey. Drizzle all the butter over the sugar topping (The butter and sugar sink through the top to make streaks of goo throughout the cake).
Bake for 35 minutes or until it looks done on top, but not too brown.
Eat warm, if you can wait that long, if not it reheats quite well.