Lay each orange down so the top and bottom of the orange (the parts with little nubs) are facing outwards. Cut each orange into thin slices. You can leave the peel on or very carefully slice it off beforehand.
Mix the sugar and orange juice or water to form a slurry. Set aside.
Preheat the oven to 180C/356F. Lightly grease and line a 20cm cake tin.
Pour the slurry into the base of the cake tin, then arrange the slices of orange in the lined baking dish. Start with one piece in the centre, then arrange them overlapping in a swirl pattern around the first piece. See the pictures above for a visual explanation.
Make sure the base is completely covered. I like to do 2-3 layers of orange and slice small pieces to fit in any gaps. This ensures no cake batter will leak through and ruin the aesthetic. Once you're happy, set the cake tin aside.
Combine the flour, sugar, baking powder, baking soda and salt in a large mixing bowl. Whisk to combine.
In a smaller mixing bowl, whisk together the milk and yoghurt, then add the vinegar. The mixture should thicken almost straight away.
Add the yoghurt mix, eggs and oil to the flour and whisk until completely smooth. Add the optional flavourings here, if you're using them.
Transfer the batter into the cake tin. Pour and spread it gently to avoid disrupting the oranges on the base. Bake for 50-60 minutes or until the base is golden brown, a skewer comes out clean and there is no wobble to the. cake. The centre middle is the last to cook, so make sure you check it is done or else it might sink on removal.
Once cooked, allow to cool for 5-10 minutes before running a knife gently around the edges. Place a serving plate over the mouth of the cake tin, then flip to invert. Tap on the base of the cake tin to ensure everything has slid off. Pull away the cake tin gently, then the piece of baking paper. You should have a beautiful, glossy upside down orange cake.
Serve warm or cool. The cake keeps well for 2-3 days in an airtight container.