Whenever I make hot chocolates with cocoa, I always mix it with the sugar before adding the water/milk. This way the sugar dissolves and leaves spaces within the cocoa, creating more surface area and leaving it unable to clump.
What sweetener are you using?
>100g Pure Vanillin powder
Wait, you mean <-? That's awesome! I figured it would be a gas or a volatile liquid though. What colour is it and how much did it set you back?
But yeah, don't mix 'em unless you really don't trust yourself to get it right every time. If that's the case then just experiment because only you know how much vanilla you want and how strong that batch of cocoa is.