Preheat oven to 350°F (180°C) conventional (see note 3 if using a convection oven with a fan). Grease two 8-inch cake tins (I use my homemade cake release) and line the bottoms with parchment paper as well, as this cake tends to stick to the bottom. Sift together flour, cornstarch, baking powder and salt. Mix together using a whisk and set aside.
In a large bowl add butter, vegetable oil, sugar and lemon zest. Using a hand or stand mixer on a medium speed, cream together for 3 minutes, until light and creamy.
Add the eggs one by one, mixing well between each addition.
Add 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 remainder of the batter will be finished by hand.
Add your pre-sifted dry ingredients to the wet mixture and, using a spatula, fold together until just combined. Do not overmix.
To finish off, toss the raspberries in 1½ tablespoon of flour (this will prevent them from sinking to the bottom of the cake layers). Then add the berries to the batter, discarding any leftover flour, and fold them in with a spatula until just combined. Again, try not to overmix.
Distribute the batter evenly into the two 8-inch cake tins, trying to distribute the raspberries evenly. Bake for 38–40 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, allow the cake layers to cool in the cake tins for about 20 minutes. Then carefully run a thin knife along the edges of the pans and turn them out onto a wire rack to cool completely. The cake layers are soft and delicate, so handle them carefully.