Preheat the oven to 350 degrees.
Grease a 7" x 11" baking tray with butter or cooking spray.
In a medium bowl, combine the dry ingredients... flours, cornstarch, baking powder, baking soda, and salt
In a separate bowl, combine the room temperature butter and sugar on high speed until well combined.
Pour the sweetened condensed milk can into the creamed butter mixture.
Add vanilla and vinegar.
Blend for a few seconds on medium-high speed until well combined. Now add flour and water in 3 to 4 parts, starting and ending with the dry. Add 1/3 of the flour, and blend on low until well incorporated.
Now add the water. Blend until well combined. DO NOT OVERMIX. Be extra careful since this is an eggless cake.
Once the batter is ready, split the batter in half. Add cocoa powder and whole milk to half the batter.
Pour the batter into a baking dish alternating between vanilla and chocolate. Simply pour a little of the vanilla, then chocolate, then vanilla, and so on.
Using a knife, form little swirls in the cake. Place in the oven for 30-35 minutes, or until golden brown. I baked my cake for 33 minutes.
Once baked, cut into 24 squares.