Preheat the oven to 325ºF. Grease a round cake pan with butter. Line the base of the pan with a round piece of parchment paper.
Mix the brown sugar and 2 tablespoons of water and form a thick paste that will go at the base of the pan on the paper. Use a spoon to smooth it out into an even layer.
Place the orange slices at the bottom of the pan, on the brown sugar layer.
In a bowl mix the ricotta, olive oil, and vanilla extract and add the eggs one at a time, and beat.
Sift the cornstarch, baking powder, and salt on the wet mixture. Mix with a spatula and add the sugar and the orange zest to the mixture.
Stir until the dough is well mixed. It should look thick and grainy. Once mixed, pour into the pan and spread evenly.
Bake 45 minutes or until a toothpick inserted in the center comes out clean (check at 40 minutes, but could take up to 1 hour).
Let the cake cool in the pan for 5 minutes and gently run a thin knife around the edges to loosen. Place a dish on top of the pan and flip it to release the cake.
Make sure the top has a nice caramel color, but not burned. Let cool completely, slice and serve.