It's important we acknowledge failures and, oh wow, did that attempt at chocolate fail hard.
The mixture had a horrible grainy texture to it no matter how much I stirred so I thought "let's add some more cocoa butter" and no, didn't help. So I checked google and came across "seized" chocolate as a thing that can happen which produces a "grainy texture" and okay, seems sensible, add a bit of boiling water and stir to dissolve everything.
"Seized" chocolate is what it instantly turned into, essentially it looks more like something you'd bake than melt and was much thicker and harder to stir. A bit more water to check and then the whole scheme was abandoned for tonight. We'll try again another day.
I don't know what I got wrong, but I'm hoping the folks over at /r/AskCulinary can give me some pointers for next time!