Preheat oven to 180 °C (350°F) conventional (see note 2 if using a convection oven with a fan) and grease two 8-inch cake tins (I use my homemade cake release). Also, line the bottoms with parchment paper. Sift together flour, cornstarch, baking powder and salt. Using a whisk, mix until well combined and set aside.
In a large bowl add in butter, vegetable oil, sugar and lemon zest. Using a hand or stand mixer on medium speed, cream together for two minutes until light and creamy.
Add in eggs one by one, mixing well in between each addition.
Add the milk, yogurt, and lemon juice. Mix until well combined—the batter may look curdled, but that's okay. Now, set your mixer aside, as the rest of the batter will be finished by hand.
Add your pre-sifted dry ingredients to the wet mixture. Using a spatula, gently fold until just combined—do not overmix. Finally, add the poppy seeds and mix until just combined. Again, avoid overmixing.
Distribute the batter evenly into the two 8-inch cake tins, and bake for 33 minutes or until a toothpick comes out clean or with a few moist crumbs on it.
Once baked, allow the cake layers to cool in the cake tins for about 15-20 minutes, and then turn them out onto a wire rack to completely cool.