I think your main issue was unit conversion, particularly for the flour (you had too much) and the chocolate chips (too little).
1 cup Nutella 300 grams[1]
1 cup brown sugar 200 grams
1 tsp baking soda
1 large egg
6 tablespoons flour 45 grams flour
1 cup semisweet chocolate chips Appx 150-175 grams depending on reference site
[1] How much does a cup of Nutella weigh?
I base the flour calculation on (6 / 16 tablespoons / cup) * 4.25 oz canonical cup for US recipes * 28.3 grams / ounce. The cup weight for flour can vary but I find 4.25 oz is a good value for recipes that don't otherwise specify.
Given that you have too much flour, probably insufficiently hydrated (the egg yolk and nutella are the only sources of water in the recipe, and I don't know how much water is in nutella), you can try adding a little more egg yolk or water to see if it helps, but your best bet would be to start a fresh batch.
Perfect black rice in a rice cooker - I just cooked 2 cups of black rice, after a single quick rinse using a wire strainer in my Zojirushi rice cooker (fuzzy logic). I used a standard American measuring cup. I added 3 and 3/4 cups of water (vs. the standard straight 2 waters for every one unit of rice - I do this for ALL rice types as I like a fully intact rice kernel with texture remaining - that fluffs and separates as you expect ).
I also added a crumbled organic vegetarian bullion cube (just one) it adds just a little seasoning (still neutral) but I think also provides a little coating on the finished rice that makes it fluff and separate better.
The rice came out perfectly - tender, with just the right kernel bite, fluffed and separated as expected.
One other note, I NEVER use the keep warm function and always pop open the cooker at the first bell signaling cooking is complete, unplug, and I fluff immediately - I leave the cooker open a minute or two to allow steam to escape, then I close it. The rice stays perfectly warm after that for a very long time, long enough for a leisurely meal and warm rice for second helpings.
I can't say enough great things about the Zojirushi rice cooker. It is worth every penny. In addition to rice it cooks wheat berry, rye berry, millet , barley, and many other grains ( including perfect oats) automatically ith zero fuss. I have owned two in 22 years and replaced the first only because the bowl was list and the model was discontinued so no replacement bowel was found.
Many blessings in your rice cooking journey!
Best Answer
In case anyone else was wondering too, I tried it and it worked fine. It's not exactly the same consistency as when using rice but I still enjoyed it very much.