My cocoa powder won't mix with melted butter I was trying to mix cocoa powder into melted butter but instead of mixing in it had a grainy texture and wouldn't dissolve. The butter was still hot and it was only melted not boiled. 

I use Heintz cocoa powder which I have used before with no problems. 

I am however using a new kind of butter. Can increased moisture content or fat content in the butter cause this?
As others have noted, cocoa powder doesn't "dissolve"; it just becomes an emulsion in the carrier. Changing the water/fat content of the carrier could certainly change whether the cocoa particles stick together or spread out, changing whether they emulsify.