Hey Des ... I make my OMM a little different than other people, so you might want to do some searches to see how others do them. I've tweeked it quite a bit.
I spray a Pyrex cup with I Can't Believe It's Not Butter a few times. Then I add an egg white, 1/2 tsp baking powder, 1/4 cup flaxmeal, and usually some cinnamon (for muffins or pancakes) or some dill seasoning (for bread). For this cake, I've added a little Torani syrup in the past. Then I mix it up really well and add some water. If it sits, it thickens up a lot, so I usually keep adding water until it's how I like it. Then I cook it for 2 minutes in the microwave. But like I said, others use cream, whole eggs, and other stuff in their OMM, so there's all sorts of ways to make them. My frosting was made from 2T of whipping cream, unsweetened cocoa, and a little sweetener.
Oh, and for the cocoa powder for the cake, I found some by Western Family that's 0 carbs. If you don't have that brand then check all the "store brands" you can find. Hersheys unsweetened cocoa powder has something like 2 or 3 carbs per serving, but some of the store brands seem to be zero.
Good luck and congrats on getting back on track!