Preheat the oven to 180 °C (350 °F) conventional. Grease a 9-inch springform pan thoroughly and line the base with parchment paper. Combine the flour, cornstarch, baking powder, and salt by sifting them together into a bowl, then whisk lightly to blend and set aside. In a large mixing bowl, cream the butter, vegetable oil, sugar, and lemon zest using a hand or stand mixer on medium speed for approximately 2 minutes until pale and fluffy. Incorporate the eggs individually, mixing well after each addition. Pour in the yoghurt, lemon juice, and vanilla, mixing gently until fully integrated, then set the mixer aside. Using a spatula, fold the sifted dry ingredients into the wet mixture gradually until just blended; take care not to overmix. Coat half of the blueberries in one tablespoon of flour to prevent sinking, fold them gently into the batter, and stir until just combined. Pour the batter evenly into the prepared pan and scatter the remaining uncoated blueberries over the top. Gently tap the pan on the counter to release any large air pockets. Bake for about 55 minutes, or until a toothpick inserted into the centre emerges clean or with only a few moist crumbs attached. Allow the cake to cool in the tin for 20 minutes before releasing the springform ring. Let the cake cool completely before removing the base and parchment. Dust generously with powdered sugar prior to serving, and enjoy as is or paired with whipped cream or ice cream.