Set the oven to 180 C / 350 F
Line a 24 cm / 10 in tin with baking paper and grease well
Put the flour and baking powder in a bowl and mix together with a fork or whisk, set aside
Zest the three oranges leaving the flesh aside for a minute. Put the zest, fine sugar (1 cup), salt and vanilla into the bowl of an electric mixer with the paddle attachment and give it a good mix so that the oils come out of the zest flavouring the sugar.
Add the eggs and beat till nice and light. Next pour in the oil and continue beating till the mixture is light and thick (about 8 minutes). This makes an airy base for the cake.
Take the bowl off the mixer and add add the milk and flour mix and fold through till combined. Leave this to rest while you continue.
Cut all of the pith (white) off the 3 zested oranges and cut into thick rounds. You should get about 4 from each orange cutting around the equator. Put the rounds into the base of the lined tin until the bottom is covered. Don't overlap them.
Pour the batter in to the tin and put into the on the middle shelf.
Bake for 40 minutes, then cover with a piece of foil and cook another 20 minutes or until a skewer inserted into the centre comes out clean
Leave to cool while you prepare the syrup.