Set a saucepan with the coffee and cardamom over a medium heat and toast for 2-3 minutes, until fragrant. Remove from the heat and pour in the cream. Set aside and leave to infuse for 30 minutes.
Meanwhile, finely chop 400g chocolate and put in a large heatproof bowl with the butter and seeds from the vanilla pod. Return the infused cream to a medium heat, bring to a simmer, then strain through a sieve onto the chocolate (discard the coffee and cardamom). Leave to stand for 1 minute, then stir until the chocolate has melted.
Beat with an electric hand whisk for 4-5 minutes, until the mixture is thick and leaves a trail when the whisk is lifted. Transfer to the fridge overnight, until softly set.
Grate or very thinly slice the remaining white chocolate into shavings. Transfer to a shallow dish.
With cold hands, gently roll the mixture into 24 small truffles. Coat in the chocolate shavings and transfer to the fridge for 1 hour, or until ready to serve or gift.