I have been told you can make a mud cake substituting cooked quinoa for normal wheat flours. Is this possible and do you need to add anything else?
Making substitutions to baking usually requires a bit of experimentation, so unless you're set on converting a particular recipe, your best option is to start with a recipe that is designed to use it. There are tons of chocolate quinoa cake recipes on the internet.
If you are hoping to convert a favourite recipe to gluten-free (the usual point of these cake recipes), then the proportions of the recipes I looked at are generally similar to flour cake recipes (2 cups cooked quinoa for a 2-layer cake), but the cake batters are mixed in a blender or food processor instead. I assume this is to make the quinoa finer, more similar in texture to flour.