Heat your oven to 350F. Spray 2 round cake pans with vegetable spray.
In a medium bowl, pour the the flour, cocoa, baking soda, and salt. Stir to combine.
In a mixing bowl, combine the vegan butter, oil and sugar and mix on medium speed until smooth. Add remaining wet ingredients and mix again until creamy.
Add dry ingredients to wet ingredients and mix again until well combined. Pour batter into prepared cake pans.
Bake for 20 - 25 minutes, until toothpick inserted in the middle of the cake comes out clean. Once done, remove the cakes from the oven, and set aside to cool for 10 minutes, before inverting onto plates or wire wracks, to cool completely.
For the Fluffy Filling: Combine the milk and flour in a saucepan over medium heat. Bring to a boil while stirring. The mixture will thicken. Once it's very thick, transfer to a lidded container, then cover and refrigerate until completely cool.
In a mixing bowl combine the vegetable shortening, sugar, and salt. Beat on high until the sugar is dissolved. Rub a little bit of it between your fingers, if it's grainy, the sugar has not dissolved yet. This may take several minutes.
Once the sugar is ready, add the vanilla and the cooled flour mixture, beating until smooth and creamy.
Spoon about half of the fluffy filling over the bottom cake, spreading evenly across the top. Then place the second cake on top of that. Spread the remaining fluffy filling evenly over the top cake.