Set the oven to 425°F and lightly coat an 8×8-inch baking pan with non-stick cooking spray.
In a small bowl, combine oat milk and lemon juice. Let it sit for 5 minutes to curdle slightly. In a large bowl, whisk together vegan mayonnaise, the milk mixture, and melted butter.
In a separate bowl, whisk together the cornmeal, flour, baking powder, baking soda, and salt.
Add the dry ingredients to the wet ingredients and whisk just until combined. Don’t overmix—just a few turns of the whisk are enough.
Let the batter rest for 10 minutes to allow the cornmeal to absorb moisture. Pour into the prepared pan and bake for 18 minutes, or until a toothpick inserted in the center comes out clean.
Cool in the pan for 10 minutes before cutting into squares. Serve warm.