Preheat the oven to 340°For 170°C. Lightly brush your springform pan with vegetable oil, then line the bottom and sides with parchment paper. Set aside.
Add the wet ingredients to a large mixing bowl and whisk well.
Sift in flour and baking powder and mix with a hand mixer or whisk for a few seconds. Do not overmix. Some lumps are ok.
Pour the cake batter into the cake pan and bake for 25 - 30 minutes at 340°F or 170°C or until a toothpick comes out dry.
Take out of the pan and let the orange cake cool down for about 30 to 45 minutes on a wire rack.
MAKE THE ORANGE CURD
Squeeze the oranges, filter them through a sift and add them to a small saucepan.Add sugar and cornstarch (or potato starch). Stir with a whisk to dissolvethe sugar and the starch.
On medium-low heat, cook the liquid for one to two minutes while stirring. When the liquid thickens, remove it from the heat and keep stirring for another minute.
FILL THE CAKE
With a serrated knife or bread knife, cut the orange cake open in half. Make sure the cake has cooled completely before you do that.
Add the orange curd and spread itevenly on the base layer of the cake.
Cover with the top layer and sprinkle with powdered sugar.Let the cake sit for about 1 hour, so the orange curd cools down, and you can cut a clean slice.If you don't want to wait, it's perfectly fine, we never do, but you won't get a clean-cut slice.