Preheat the oven to 400°F. Line a 12-hole muffin pan with red paper liners.
Beat the eggs and sugar with a mixer on medium-high speed until the mixture is frothy and a pale yellow, then beat in the vanilla and orange zest.
Pour the oil and half of the milk into a small jug with a spout, and gradually trickle it into the egg mixture. Beat on medium-high speed until all the liquid is incorporated.
Sift the dry ingredients (flour, cornstarch, baking powder, baking soda, and salt) together into a separate bowl. Sift them again into the beaten egg mixture and beat on medium speed until blended.
Add the rest of the milk and mix on medium-high speed until the batter is very smooth and free from lumps.
Pour the batter into a jug with a spout and fill each cup three quarters full with the batter.
Just before putting the muffins in the oven, lower the temperature to 350 F and bake for 20 minutes, until a toothpick inserted into the center comes out clean.
Remove them from the muffin pan and let them cool on a wire rack for at least 15 minutes before eating.