Preheat the oven to 350 degrees and line a cake pan with parchment paper.
In a small bowl sift the cassava flour, arrowroot starch, and cocoa powder together. Run it through at least two times. Once sifted add the salt, baking soda and baking powder and set it aside.
In a larger bowl whisk the eggs and sugar together until they are fluffy and lightened in color. See notes for egg free.
Add the milk, oil and vanilla to the egg mixutre and whisk until combined.
Mix in the flours making sure to only mix just until combined, you do not want to over mix the batter.
Once the batter comes together add in the hot water and mix until combined. The batter will be thin, it's suppsed to be.
Pour the batter into the prepared cake pan (or cupcake liners if using them) about ¾ of the way full. The batter will expand.
Bake the cake in the preheated oven for 30-35 minutes in an 8-9 inch cake pan or about 15-20 minutes for cupcakes. The cake is done when it bounces back and a toothpick inserted comes out clean, with a few crumbs.
Allow the cake to cool before frosting. See notes above for my favorite grain-free frosting recipe.