In a large bowl, combine flour, sugar, cocoa powder, salt, and baking soda. Whisk to combine.
Add vanilla, vinegar, oil, and water to the dry ingredients and stir using a spatula or whisk until ingredients are fully combined.
Pour into a prepared (greased or parchment-lined) 8x8-inch pan and bake for 30-35 minutes. When an inserted toothpick is removed with a few crumbs (not clean!) the cake is done.