In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda and salt. Set aside.
In a large bowl, with an electric hand mixer, beat the butter on medium speed until creamy, about 1 minute.
With the mixer running on medium-low speed, gradually add in the sugar, followed by both orange and lemon zest. Raise the speed to medium-high and continue beating together until lightened up in both color and texture; 4 to 5 minutes. The color should turn very pale (almost white) and texture should look fluffy
Add the eggs, one at a time, beating well after each addition. Add in the vanilla and mix until well incorporated.
On lowest speed, add in one-third of the flour mixture and mix until mostly combined. Add in half of the orange juice and lightly stir until almost mixed in. Continue with adding another one-third of the flour mixture, followed by the remaining orange juice and ending with the last third of flour mixture. Mix until just combined and a smooth batter forms. Do not overmix. With a rubber spatula, give the batter one good, final stir to make sure that everything is well incorporated. Transfer the batter to the prepared pan and smooth the top.
Bake for 35-40 minutes, or until the center of the cake springs back when lightly touched, and a wooden skewer inserted between the tube and sides comes out clean or with a few cooked crumbs attached.
In a tiny saucepan over very low heat, bring together the orange juice and sugar to a simmer until the sugar is dissolved. Remove from the heat once it reaches a simmer. Using a thin skewer, poke tiny holes all over the cake. Brush the surface of the cake with about one-third of the syrup. Let the cake cool in the pan for 10 minutes, then invert onto a serving platter. Brush the top and sides of the cake with the remaining syrup. Allow the cake to cool slightly before serving