In a small mixing bowl, add water to the raw cashews and leave them to soak for 4 to 6 hours. (This will be used for the cream filling later.)
Preheat the oven to 350° F.
Prepare a 18 x 13 inch sheet pan with parchment paper, set aside.
Measure the plant based milk in a one cup measuring cup.
Next, stir in the apple cider vinegar, and set aside to curdle, this is the vegan buttermilk.
In a small mixing bowl, mix together the flours, cocoa, baking soda, baking powder, and salt.
*Can't find oat flour? Make your own, add the oats to a high speed blender and blend until you have the consistency of a flour.
Next add the maple syrup, apple sauce, oil, vegan buttermilk, and the vanilla.
Continue by stirring until well combined, then pour and spread into to the sheet pan.
Place into the oven and bake for 20 minutes.
Once the cake is done, remove from the oven.
Leave the cake to cool for about 15 minutes or until you are able to touch.
Pick up the cake by the parchment paper, flip the pan over and place the cake on the bottom of the pan, roll a rolling pin over the cake to flatten.
After flattening the cake, pick the cake up by the parchment paper, flip the pan back over, and place the cake back to continue to cool completely.
While the cake is cooling, make the cream filling.