I ended up tweaking the recipe a bit by adding about 20g more flour to each dough mix since they were way too sticky to roll, even after being in the fridge for thirty minutes. No fault of the actual recipe though - our scales are dodgy and we usually add at least 10g extra for each ingredient!
I might use proper cocoa powder instead of hot chocolate mix next time and fling in some extra sugar to account for the bitterness...Or perhaps add orange extract instead of vanilla and use dark chocolate...All in all, I definitely give Danni's recipe a big thumbs up - try it yourself!
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