Cocoa Powder in Yeast Bread

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I have a go-to soft and fluffy milk bread recipe at 74% hydration that stays moist up to 2-3 days after. I wanted to make a chocolate version so I substituted 20g of the bread flour with Dutch-processed cocoa powder, other ingredients and baking process were kept constant. 

While they turned out really yummy, I noticed this version dries out much faster compared to the plain milk bread. What could be the reason for this and are there any adjustments I can make so that it stays moist longer like the plain one?

Thank you! 

Several ways to go about it, either add an egg yolk, or a tablespoon or two of butter or coconut butter, or oil.  Cocoa is very dry.  One can also blend the cocoa with just enough applesauce to make a thick batter skipping the fat addition and then blending with the liquids.

You’ll also find that the tannins in cocoa tighten the gluten so increasing the hydration is useful to help counter that. 
Benny