I'm thinking that the addition of cocoa powder to the recipe in addition to the flour may add too much density, creating a dryer muffin. I suggest cutting the flour back to 1-1/2 cups and then adding the 2/3 cup cocoa powder. The milk in the mixture is more than adequate. Your altitude won't make a difference in this recipe.
Your oven may bake hotter, however. To gauge it, place an oven thermometer in their to determine its accuracy and then adjust as needed. If all else fails, simply devour the muffins dipped in milk!
Hope this helps,
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