Preheat the oven to 350°F (180°C). Grease and line an 8x8-inch (20x20 cm) square baking pan with parchment paper, leaving some overhang for easy removal.
In a large mixing bowl, combine the milk and lemon juice (or vinegar). Stir and let it sit for 5–10 minutes until slightly curdled, creating a homemade buttermilk.
Add the sugar and vanilla extract and whisk until combined.
In a separate bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt.
Add the dry ingredients to the wet ingredients and gently whisk until just combined. Pour in the melted butter and mix until you have a smooth, lump-free batter. Be careful not to overmix, as this can result in a dense cake.