Set your oven to 350°F (175°C). While it’s heating, grease two 8-inch (20 cm) round or three 6-inch (15cm) round cake pans with oil. Line the bottoms of each pan with parchment paper to prevent sticking.
In a medium bowl, pour in the room temperature milk and add the vinegar. Stir gently, then let it sit for 5 minutes. The mixture will curdle slightly—this is your homemade buttermilk.
To the curdled milk, add the sugar, oil, and vanilla extract. Whisk everything together by hand until the sugar is mostly dissolved and the mixture looks smooth and slightly glossy.
In a separate large bowl, add the flour, cornstarch, baking soda, baking powder, and salt. Mix well, then sift the dry ingredients directly into the wet mixture. This helps prevent lumps and ensures an even batter.
Gently whisk the wet and dry ingredients together until you get a smooth, lump-free batter. Avoid over-mixing—stop as soon as everything is combined.
Divide the batter evenly between the two prepared cake pans. Use a spatula to spread the batter evenly and smooth the tops.
Place the pans in the center of the oven and bake for 25 to 30 minutes. The cakes are done when the tops spring back when lightly pressed or a toothpick inserted into the center comes out clean.
Remove the pans from the oven and place them on a wire rack. Let the cakes cool in the pans for about 10–15 minutes, then carefully run a knife around the edges and turn them out onto the rack to cool completely.