Preheat the oven to 350° F (175° C).
Melt the coconut oil and cacao paste together in a double boiler over medium-low heat.
Whisk the eggs and vanilla extract in a medium mixing bowl.
In a separate bowl, combine Lakanto, cacao powder, psyllium husk, and salt.
In an additional bowl, mix the baking soda and vinegar. Let it fizz up and set aside.
Combine the melted oil and cacao into eggs. Mix in the dry ingredients. Add the baking soda and vinegar. Continue mixing, smash out any lumps. Finally, add the boiling water and stir until the batter completely smooth.
Grease an 8 by 8-inch glass baking dish. Transfer the batter in the pan, make sure it is evenly spread. Bake for 30 minutes, until the center is firm and the edges pull away from the pan. Test by placing a toothpick in the center and check that it comes out clean.
Cool before cutting. You may dust with powdered sweetener if desired.