a round loose-bottomed 9-inch (23-centimeter) cake pan
Preheat the oven to 350 degrees F (180 degrees C). Grease a round loose-bottomed 9-inch (23-centimeter) cake pan with cooking spray and line the bottom with parchment paper.
In a small bowl, mix the brown sugar and water to make a thick paste. Spread this thinly and evenly onto the base of the cake pan and set aside.
Zest 1 orange and add this to a large bowl with the superfine sugar. Rub the zest into the sugar with your fingertips until the zest is evenly distributed. Set aside.
Slice both oranges as thinly as you can with the skin on and arrange the slices on top of the brown sugar base in the cake pan. You can cut some of the slices in half to create a different pattern and to fill in all the gaps.
Add the yogurt to the bowl with the superfine sugar and whisk briefly to combine. Whisk in the eggs and olive oil. Stir in the flour, baking powder, baking soda and salt until smooth. Gently pour the batter on top of the orange slices, being careful not to disturb the oranges underneath.
Bake for 30 to 35 minutes, or until the sponge is firm to the touch and golden brown. Leave the cake in its pan to cool completely before attempting to turn it upside down. Serve with creme fraiche.