Step 1

Position an oven rack in the middle of the oven and preheat to 350 degrees F. Grease an 8-by-8-inch baking pan with vegetable oil.

Step 2

Sift the flour, sugar, baking soda and salt through a fine-mesh strainer directly into the prepared baking pan. Stir together with a fork (or a silicone whisk if using a nonstick baking pan). Add the oil, vinegar, vanilla and 1 cup water; stir with the fork until fully incorporated. Bake until a toothpick inserted into the center comes out clean, about 35 minutes.

Step 3

Let the cake cool completely in the pan on a wire rack, about 1 hour. Frost the top of the cake with the frosting, if using. Cut into 9 squares and serve each with a dollop of whipped cream, if using.