Spray a 9x13" pan with non-stick spray and set aside.
Bring quinoa and water to boil in a large saucepan. Reduce heat to low and simmer for 15-20 minutes, or till quinoa is soft. Let cool to room temperature. Measure 2 cups, discard the rest.
Preheat your oven to 350°.
Combine the milk, eggs, and vanilla in a blender and process till just combined. Add the 2 cups of cooked quinoa and the melted butter. Process until very smooth.
In a large mixing bowl, whisk together the sugar, cocoa, baking powder, and baking soda. Add the quinoa mixture and stir till well combined.
Pour batter into the pan. Bake for about 35 minutes.
Cool completely, then frost.
For frosting, beat all ingredients till smooth, adding enough milk to achieve a spreadable consistency.
Notes
To make a layered cake, prepare two 8" round pans by lining with parchment and spraying with Baker's Joy. Bake for only 28-30 minutes. Let cool in pans 5-10 minutes, then carefully invert onto cooling racks. Wrap in plastic wrap while still hot and freeze for several hours. (Cake is fragile, it is much easier to handle when frozen.)
Two to three hours before serving, place one frozen layer on a cake stand. Frost the top. Place the other frozen cake on top, frost sides and top. Let cake sit at room temperature till thawed.
You can use chocolate frosting, or buttercream for the filling and sides and chocolate ganache for the topping. If you use my frosting, you will need 1 1/2 - 2 recipes to cover the whole cake.