There totally is. I recommend using MALK almond milk for it and pure cocoa powder with a zero cal sweetener and boil it all together (mixing). Supposedly, it doesn't get that creamy otherwise. Then marshmallows and other seasoning if you want (cinnamon, nutmeg, etc)
This wil help you calculate that but as long as you measure everything out it should be fairly accurate. You can alao calculate the entire things calories, distribute evenly, and divide by how many servings there are.
For example with only a cup of almond milk, cocoa powder, and mini marshmallows (+stevia) this is it. However I would calculate this specifically by not including the cocoa, and instead googling how many cals is in what amount you used (for pure powder you don't need much) then add it to this
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https://caloriecontrol.org/healthy-weight-tool-kit/food-calorie-calculator/