The mint-choc flavour boost is much more chocolatey than the mint-choc premix, which tastes more like mint choc-chip ice cream than minty chocolate.
I’d actually recommend adding real cacao powder to the mint choc premix as this works really well and tones down the mint and the sweetness and adds chocolateyness without making it sickly.
As for the difference between the chocolate flavour boost and the cacao flavour boost - they are suprisingly different.
The chocolate tastes more like milk chocolate - like a bar of Cadbury’s almost.
The cacao tastes more… well… chocolatey! And less milky. It doesn’t taste like dark chocolate exactly, but more like rich chocolate.
Of the two, I prefer the cacao boost - it is delicious, especially in 50:50 Original/Unflavoured
Adding the chocolate or cacao flavour boost to the mint-choc premix makes it far too sweet in my opinion.
So cacao powder (or cocoa powder) is definitely the solution