OK so I found that recipe you tried--I think those directions are confusing. Adding that much milk and water to the butter-sugar-egg would make a real mess.
So that is a recipe from a famous bakery---no reason for them to give away all their secrets is there?
I make lots of recipes where you mix the cocoa with boiling water, it turns into a thick paste. When it cools you add it to the batter and those cakes are always super good. That might be part of what's missing.
So--about your cracking tops with other recipes, you are getting too much flour into your measuring cup.
This recipe has a button to give you WEIGHTS of ingredients. Try weighing at least your flour to get the right consistency. It also tells you that the cakes are supposed to spread a fair bit when they bake.