Preheat oven to 350 °F
Line a 8 by 8 inch baking dish with parchment paper
In a large mixing bowl, combine Flour, Cocoa, Sweetener, Baking Powder, Baking Soda and salt. Whisk together.
Add the sour cream, oil and vanilla and mix well.
Pour the boiling water, mixing slowly as you go.
Pour the batter into the prepared pan and smooth it out evenly.
Bake for 10-12 minutes, or until the cake feels springy to the touch.
Remove from oven and all to cool completely. Chill in the fridge if possible.