Preheat the oven to 180C/350F/gas mark 4. Have ready a 25cm solid-bottomed cake tin or skillet that can go into the oven.
Melt 50g butter and demerara sugar in the pan and swirl around to coat. Split the vanilla pod, scrape the seeds into the butter and break up with a fork. Lay the split vanilla pod in the bottom and remove from the heat.
Take the supremed oranges and slice them crossways into 1.5cm slices. Arrange the slices in the bottom of the pan on top of the vanilla pod.
Put the flour, salt and spices into a bowl and whisk them together well. Set aside. In a small saucepan, melt 200g unsalted butter, the sugar and treacle. Whisk together until smooth. Carefully mix this into the dry flour mix.
Whisk in the milk and finally the eggs. Pour the cake mixture over the oranges and bake in the oven for 45-55 minutes or until springy and a skewer inserted comes out clean.
Let the cake cool for 15 minutes in the tin. Then use a knife to release the cake from the edge of the tin. Put a plate over the cake and turn it upside down so that the bottom becomes the top.