Heat oven to 350°F. Get out a 9x9" baking pan, and set it aside (there is no need to grease it).
In a large bowl, whisk together 1 1/2 cups all-purpose flour, 1 cup granulated sugar, 2 Tablespoons cornstarch, 1 teaspoon baking powder, 1 teaspoon baking soda, and 1/4 teaspoon table salt.
Using the back of a spoon or a small measuring cup, make 3 indents into the flour mixture. Pour 3 Tablespoons canola oil, 1 Tablespoon white vinegar, and 2 teaspoons vanilla extract into each of the indents. Pour 1 cup lukewarm water over the ingredients in the mixing bowl, and mix with a spatula until no dry bits remain.
Pour the batter into the 9x9" baking pan, and bake for 30 minutes, until a toothpick comes out with a few crumbs clinging to it. Cool completely before frosting.
Easy Vanilla Buttercream (optional)
Add 1 1/2 cups powdered sugar, 3 Tablespoons unsalted butter, 1 Tablespoon milk, 1/2 teaspoon vanilla extract, and 1 pinch salt to a medium bowl. Using a hand mixer on low speed, mix until the powdered sugar has been incorporated. Increase the mixer speed to high, and keep mixing until the frosting is light and fluffy. sugar
If you want the frosting to be stiffer or sweeter, add an additional 1/2 cup powdered sugar, and mix again (starting with the mixer on low speed, and increasing to high after the sugar has been incorporated).
Spread the frosting evenly over the cooled cake and cut into squares to serve.