Preheat the oven to 350 F and grease a cake pan. Put a kettle of water on to boil.
Melt the cacao butter and coconut oil together in a saucepan over low heat, stirring occasionally.
Once fully liquid, stir in 1/2 cup of the cacao powder. Set the remaining 1/4 cup aside. Remove from the heat.
In a large bowl, whisk the eggs and vanilla extract together.
In a second bowl, combine the sweetener, the reserved cacao powder, the psyllium husk and the salt.
In a small bowl, stir the baking soda into the apple cider vinegar and let it fizz.
Pour the warm cacao and oil mixture into the eggs in a slow stream, whisking constantly so the eggs do not scramble.
Whisk in the dry ingredients until smooth, then add the fizzing baking soda mixture.
Pour in the boiling water last and whisk until the batter is completely smooth. It will look thin, which is correct.
Pour into the prepared pan and bake for 30 minutes, until the center is firm and a toothpick comes out clean.
Cool in the pan for 10 minutes before slicing.
