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Beat the butter and sugar together in a stand mixer until well combined, then add the vanilla and flour. Beat in the finely chopped dark or milk chocolate. Roll spoonfuls of the dough into balls using your hands (about 15g each), then transfer to a tray or plate and chill for at least 1 hr. You should have about 30 balls.
Put the remaining dark or milk chocolate in a heatproof bowl and melt in the microwave in short bursts. Lower each truffle into the melted chocolate using a fork, coat well and put on a tray lined with baking parchment. Chill for 1 hr. Melt the white chocolate as you did the dark, then drizzle over the truffles. Sprinkle with a little sea salt, then chill for 30 mins more, or until ready to eat or package. Will keep in the fridge for up to one week.