Preheat the oven on to 175C (347F). Line a the bottom of a 25cm (10inch) round cake with baking paper tin and spray with cooking oil.
In a large mixing bowl, cream the butter and brown sugar together using a stand mixer or electric beater until light and pale. Add the orange zest and beat until incorporated throughout the batter. This should take a few minutes. Add the eggs one by one and beat well with each addition.
Prepare the dry ingredients. In a medium sized mixing bowl, sift the plain flour and the baking powder together. Add half of the dry ingredients and add half of the yoghurt to the butter mixture. Fold the ingredients together gently and stop as soon as it just comes together. Add the remaining dry ingredients and yoghurt and repeat. Set aside.
In a small bowl, mix the orange zest, melted butter and brown sugar together. Put the sugar/butter mixture into the prepared cake tin, on top of the baking paper. Arrange the sliced oranges on top of the sugar/butter mixture. Top with the orange cake mixture and carefully spread the cake mixture evenly into the cake tin. Bake for around 40-45minutes or until a skewer comes out clean.
Let the cake cool until warm and then invert the cake onto a plate larger than the cake tin. Let the cake sit for a minute for the juices to run and seep into the cake. Carefully remove the baking paper and serve immediately. Store an airtight container at room temperature for up to 2 days and in the fridge for up to 5 days. Warm slightly before serving again.