Put 200g sugar and 120ml water in a large saucepan and place over a medium heat on the hob. Slice your oranges into 3mm rounds and add to the pan. Bring to a gentle simmer, and leave to bubble for 15 minutes until the oranges have softened slightly. Remove from the heat.
Preheat the oven to 180C / fan 160C and grease a 20cm round cake tin. Line with parchment paper, then arrange the orange slices on the bottom, starting from the centre and working your way outwards, overlapping the rounds. Drizzle over a tablespoon or two or leftover sugar syrup and set aside. N.B. Keep your orange sugar syrup for later. Sift the flour and baking powder into a bowl then add the orange zest and salt. Set aside.
Beat the softened butter and remaining 180g sugar with an electric whisk for 5 mins until pale and creamy. Add the eggs, one at a time, mixing well between each addition, then gently mix in the flour mix until just combined. Be careful not to overman Once incorporated, stir through the milk.
Pour the batter into the cake tin, over the oranges slices, then smooth the top. Bake for 40 mins until the cake is golden, and a skewer or cake tester inserted into the centre comes out clean.
Leave to cool in the tin for 5 mins, then run a knife around the edge of the cake to loosen it slightly. Invert immediately onto your plate, peel off any parchment paper to avoid it sticking to the sugar syrup and leave to cool completely.
Transfer to a wire rack, peel off the paper and drizzle with another 2 tbsp orange syrup. Leave to cool completely, then serve with the extra syrup or a dollop of crème fraîche, ice cream or cream.