Preheat oven to 160 °C (320°F) with the fan on (see note 3 if you don't have a fan function) and line a cupcake tray with cupcake liners.
In a bowl, sift together your flour, cornstarch, baking soda, baking powder and salt. Using a whisk or fork, mix until well combined. Set aside.
In another bowl, add in your butter, vegetable oil, sugar and lemon zest. Using an electric mixer (hand or stand mixer are both fine - see note 4), cream together for 2 minutes until light and creamy.
Add in eggs one by one, mixing well in between each addition (about 10-15 seconds between eggs).
Next add in Greek yoghurt and fresh lemon juice. Mix until well combined. The batter may look a little curdled at this stage but that's okay.
Lastly add in your premixed dry ingredients from earlier, and using a spatula, gently fold until just combined. Do not overmix. Your batter should no longer be curdled, and there should be no large chunks or streaks of flour. It's okay if the batter is a little lumpy in the end.
Distribute the batter evenly into the cupcake liners (fill no more than about 3/4s of the way up), and bake for 18 minutes or until a toothpick comes out clean. Allow to cool in the tray for 10 minutes, before transferring to a wire rack to completely cool. The cupcakes do ever so slightly sink in the middle while cooling , but once you fill and frost them you won't be able to tell. In my opinion, it's a necessary trade-off for the perfect texture :)