For the orange topping: Firstly, grease and line the base of a 33x22cm baking tin and set aside. Then thinly slice the oranges, cutting off the peel but keeping a thin edge of the white pith to keep the slice intact. Lay the slices in a single layer in the prepared tin, keeping them tight together. Then sprinkle over the 50g caster sugar, and set aside.
For the cake: Preheat the oven to 180C/160C Fan/Gas Mark 4. In a large mixing bowl, cream the butter and sugar together using an electric hand-whisk until light and fluffy. Then add the vanilla and eggs, and whisk again until combined. Next add the flour, baking powder and salt, whisking again until just combined. Finally, whisk in the milk until just combined. The batter will be very thick.
Spoon the cake batter onto the sugared orange slices, and spread evenly using a spatula or spoon. Then bake in the oven for 27-30 minutes, until a skewer or cocktail stick inserted into the middle comes out clean. Set the tin onto a cooling rack for 10 minutes.
Then turn the cake out onto the cooling rack, so the orange slices are now on the top. Carefully, peel off the baking paper and leave to cool completely. Then serve.
Best eaten same day. Can be kept in an air-tight container for 1-2 days.