Preheat oven to 355˚F (180˚C) and grease an 8-inch (20cm) cake pan. (You can line it additionally with parchment paper for easier removal).
Sift the flour, cornstarch, baking powder, and baking soda into a large bowl. Add salt and sugar and whisk to combine.
Add the oil, sparkling water, vanilla extract, lemon zest, and vinegar. Stir until just combined. (Please do not overmix or the cake will get dense instead of fluffy. It’s okay if the dough looks a bit lumpy instead of super smooth).
Fill the dough into the prepared cake pan and bake for approx. 35 minutes, or until a toothpick inserted in the center comes out clean and the surface of the cake springs back when gently touched with your fingertips.
Remove from the oven, allow to cool in the pan for 15 minutes. Then, carefully, invert onto a wire rack to cool completely.