Preheat your oven to 350°F/175°C. Spray 2 round 8 or 9-inch cake pans with vegetable spray. Cut rounds of parchment paper to place in the bottom of each pan and spray those with vegetable spray.
In a medium bowl, pour the flour, sugar, cocoa powder, baking soda, and salt. Stir to combine.
Stir together the water, oil, vinegar, and vanilla in a separate bowl.
Stir together the wet and dry ingredients, stirring vigorously until there are no large lumps of flour and only minimal small lumps. Pour batter into prepared cake pans.
Bake for 30 to 35 minutes until a toothpick inserted in the middle comes out clean. Once done, remove the cakes from the oven, and set them aside to cool for 10 minutes. Then invert them onto plates or wire racks to cool completely.
For the Fluffy Filling
The filling for this cake is basically a boiled flour frosting. To make it, whisk together the sugar, flour, and salt in a saucepan over medium heat. Cook for 1 to 2 minutes to toast the flour.
Pour in the vegan milk and use a whisk or spatula to stir it in.
Continue stirring as the mixture comes slowly to a low boil and thickens. Don't turn up the heat, or you'll risk burning it. Once it's thick enough that a spoon leaves a trail, transfer it to a heat-proof bowl, cover it with waxed paper touching the surface (to prevent a skin from forming), and refrigerate until completely cool.
Once the flour mixture is cool, add the butter to a mixing bowl and mix on medium-high speed until light and fluffy, around 2 minutes.
Turn the mixer speed down to medium and spoon in the cooled flour mixture, a spoonful at a time, mixing to incorporate it until adding more. You'll need to scrape down the sides of the bowl regularly.
Once all the flour mixture is added and mixed, add the vanilla, turn up the speed to medium-high, and whip for up to a minute until the frosting is light and fluffy.
When both cakes are completely cooled, spoon about half of the fluffy filling over the bottom cake, spreading it evenly across the top. Then place the second cake on top of that. Spread the remaining fluffy filling evenly over the top cake.
You can substitute vegan butter such as Earth Balance Buttery Sticks, Miyoko's, or homemade vegan butter in place of vegetable shortening. I do not recommend using butter-flavored vegetable shortening.