Preheat your oven to 350F - 180C and line a 22cm round cake pan with parchment paper (only the bottom of the pan).
Take your 3 blood oranges and remove the skin (including the thin white skin) with a sharp knife. Cut each orange into thin slices and place them all in a single layer on the bottom of your prepared pan. Place aside.
Sift the flour with salt in a bowl.
Combine the eggs with sugar and vanilla in a bowl and mix about 7 minutes on high speed until triple in volume, fluffy and pale.
Using a spatula, incorporate the flour, 1 tablespoon at a time. Mix gently, from bottom to top and make slow moves, trying not to deflate the eggs.
Take 3-4 tablespoons of batter in a smaller bowl and drizzle in the melted butter. Mix with your spatula until incorporated then transfer this mixture into the bigger bowl and mix just as gently. This is done because butter tends to sink at the bottom of the bowl so you'd have to mix into the batter for a longer time thus deflating it more and more and over-mixing it. But mixing it with a smaller quantity of batter first prevents it from sinking and incorporating it becomes easier. It's just a small trick I learnt and been using with great success for genoise cakes.
Pour the batter over the oranges and level it. Bake in the preheated oven for 35-40 minutes or until well risen and if you press the center of the cake with your fingertips, it springs back. The toothpick test can be used as well.
When done, remove from the oven, loosen the edges of the cake with a knife and turn it upside down on a platter. Carefully remove the parchment paper and let the cake cool down.
Blood orange syrup:
Mix the juice with sugar in a saucepan and bring it to a boil.
Cook the syrup for 10 minutes until slightly reduced then remove from heat and add the orange zest.
Let the zest infuse for 5 minutes then strain through a fine sieve.
When the syrup cools down, carefully pour it over the cake with a spoon, making sure it soaks the cake evenly.
Serve the cake chilled.
Recipe by Pastry Workshop at http://pastry-workshop.com/blood-orange-upside-cake-tort-intors-cu-portocale-rosii/