Preheat the oven to 350°F (175°C) and grease a pan.
In a bowl, mix flour, cocoa powder, baking soda, salt, and sweetener.
In another bowl, whisk together nut butter, yogurt, water, and vanilla.
Pour the wet ingredients into the dry ingredients and stir until it is combined.
Pour the batter into the greased pan and bake for 25 minutes.
Let the cake cool, then refrigerate it overnight.
Enjoy the cake the next day for a much more rich taste!