Preheat the oven to 350 degrees. Grease 2 9 inch cake pans with cooking spray, then line with a parchment paper round. Set aside.
In a large bowl, whisk together the cocoa powder, sugar, flour, salt, soda, and baking powder.
Make a hole in the center of the dry ingredients then pour in the water, oil, vinegar, and vanilla. Stir all the ingredients together, then whisk until smooth.
Pour in the prepared baking pans and bake for 28-31 minutes, or until the cake is starting to pull away from the sides and a toothpick inserted in the center comes out clean.
Set the pans on a cooling rack to cool for 10-15 minutes, then grease the cooling rack with non-stick spray. Run a knife around the edges of the cake pan top with the cooling rack, and carefully flip the cakes out. Allow to cool completely, then wrap and chill in the fridge overnight or freeze at least one hour. Cake layers can be frozen up to 3 months.