Butter and flour a 22cm cake tin with 5cm high sides, or butter and line with baking parchment. Sift the flour with the baking powder and set aside.
Using an electric whisk, mix the egg yolks with the sugar until they are doubled in volume. Continue mixing with a wooden or silicone spoon and add the orange juice, grated zest, oil and the flour and baking powder. Mix well.
Beat the egg whites until they form peaks. Add a little of the beaten egg whites to the orange mixture to soften it, and then add the rest of the whites, mixing from the bottom to the top. Pour the mixture into the prepared cake tin. Place the orange slices on top.
Put the cake in a cold oven and then turn on the oven to 200 C / Gas 6. After 40 minutes, check to see if the cake is done: a skewer inserted in the centre should come out clean. Turn off the oven and leave the cake inside for another 10 minutes.
Take the cake out of the oven and place it on a cooling rack for 10 minutes. Remove the cake from the tin, turn it upside-down and take off the baking parchment if it was used, and then turn it right-side up onto the rack to cool.
In the meantime, heat up the jam with 1 spoonful of water and brush it on top of the cake. The jam will help tone down the bitter taste of the oranges and keep the cake from getting too dry.
I followed this recipe exactly.It turned out to be rather disappointing. I would suggest that the recipe could be improved by stating the VOLUME of orange juice rather than the number of oranges because, of course, orange size and juiciness varies hugely. My mixture (having had large oranges) was too runny. I allowed two and a half times the baking time (with foil placed over the top) to try to get it to cook through, with limited success. The cake looked wonderful, but it's texture was stodgy. Having used expensive ingredients, I did not want to bin it, so used some of it as a pudding with custard, which worked reasonably well. - 14 Dec 2015