Preheat oven to 350°F (175°C). Grease two 8-inch (20cm) cake tins with oil spray and line it with baking paper.
In a large bowl, whisk together flour, cornstarch, baking powder, baking soda, salt, and sugar.
In a separate bowl, combine soda water, vinegar, oil, and vanilla extract. Gradually add to dry ingredients, mixing until smooth and light.
Divide batter between prepared tins.
Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
Cool in tins for 30 minutes, then transfer to a wire rack. For a firmer cake, wrap and chill in the freezer for a short time.
Once cooled, slice each cake in half horizontally, creating four layers.