Preheat your oven to 350°F (175°C). Grease two 6-inch (15cm) cake pans and line the bottoms with parchment paper for easy removal.
In a medium bowl, combine the milk, lemon juice, and yogurt. Let the mixture sit for about 5 minutes until it looks slightly curdled; this creates a homemade buttermilk that ensures a tender crumb.
In a separate bowl, rub the fresh lemon zest into the sugar with your fingertips until fragrant. Pour in the milk mixture and whisk until the sugar is mostly dissolved.
Add oil and vanilla extract, whisking until the wet ingredients are smooth and well combined.
Sift flour, cornstarch, baking powder, baking soda, and salt into the liquid ingredients, mixing until just combined.
Gently fold in the flour-tossed blueberries, being careful not to overwork the batter. Divide the batter evenly between the prepared pans.
Bake for 25 to 30 minutes, or until the cakes are set and a toothpick inserted into the center comes out clean. Allow the cakes to cool in their pans for 30 minutes before transferring them to a wire rack to cool completely.
Once cooled, level the tops with a serrated knife for even stacking. If you aren't assembling immediately, wrap the layers tightly in plastic wrap and chill in the fridge.