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Right now, I have cooked steel- cut oatmeal, water, banana, cinnamon powder, sugar, and dark chocolate in my cookie batter.
2 cups of cooked oatmeal, 1 banana, half a teaspoon of cinnamon, ~30 grams of chocolate, ~2.5 teaspoons of cane sugar, and enough water so that the batter is somewhat sticky.

When I baked it (at 375 degrees Fahrenheit for 10 minutes), however, the cookies came out as leathery, chocolate- flavored skins encasing a blend of bananas and chocolate- flavored oatmeal. It seemed as if the insides hadn't been cooked at all. I tried baking it for an additional five minutes, but doing so didn't change anything.

Why did this happen? And what should I do to fix this problem? Thanks in advance!

If you want to use steel-cut oats/oatmeal, you'll probably want to start with a recipe that calls for it. You can substitute old fashioned oats instead of quick-cook oatmeal in most (possibly all) cookie recipes but you can't substitute cooked oatmeal without making major adjustments.

In this case, old fashioned oats are specifically called out in the recipe as a substitute with the note that quick cook oats will give the best result:

• Quick oats give the best results, however you CAN use rolled oats. Use Gluten Free Oats for gluten free cookies.

Oats are a dry ingredient. There's no water in them so they absorb moisture from the other ingredients to cook. In this case, the banana. When you cook the oats first, you're introducing a lot of extra water and it's likely to make them take much longer to cook and change the texture.

In a recipe for cookies made from instant steel cut oats, I found this substitution note:

If you’re using traditional steel cut oats (not quick cooking), you’ll want to cook them first, just like when you’re making oatmeal. To use cooked oats, simply substitute one cup of cooked oatmeal for the instant raw oats listed in the recipe, and add an extra half cup of flour (or more as needed to achieve a non-runny dough consistency).

You might be able to salvage this recipe by doing something similar but if you really want to use steel cut oats, you'll get better results by finding a recipe that actually calls for them. If you want to use this specific recipe, use old fashioned or quick-cooking oats.