Heat the oven to 180C/160C fan/gas 4 and butter a 23cm cake tin. Zest the oranges, then remove the peel and pith using a small, serrated knife. Set aside. Cut the oranges along the equator into 1cm-thick slices. Sprinkle 25g sugar over the base of the tin and arrange the orange slices on top. Set aside while you make the syrup and cake batter.

Weigh the orange peels into a saucepan with the pith, and add the same amount of sugar and water (about 100g). Bring to a simmer, then boil for 10 mins until reduced and thickened. Set aside to infuse until needed, then strain.

Beat the butter and 200g of the sugar together in a stand mixer or using an electric whisk until light and fluffy, about 5 mins. Crack in the eggs, one by one, beating well after each addition. Finally, beat in the orange zest and marmalade before carefully folding in the flour and a pinch of salt. Spoon the batter over the orange slices and carefully level the surface. Bake for 35-40 mins until golden and a skewer inserted into the middle comes out clean.

Leave to cool in the tin for 15-20 mins before carefully inverting onto a plate. Brush some of the bitter orange syrup over the top (any leftover syrup can be used for cocktails). The syrup will keep chilled for two weeks. Serve warm with cream or ice cream. Once completely cool, the cake will keep in an airtight container for three days.