Gently melt the butter and chocolate in a bowl over simmering water, mixing throughly to combine.
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Take the chocolate and butter off the heat and whisk in the egg yolks, cream, vanilla, salt and optional sweetener.
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Line a pan with cling film or grease proof and pour the mixture into it. Spread it out evenly, let it cool and put it in the fridge to firm up — at least two hours.
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Once chilled and firm, cut into squares and dust with cacao. Or, if you wish you can leave them plain.
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Store the truffles in the fridge for up to 2 weeks, or in the freezer for up to 3 months.
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Remove from the fridge, 15 minutes before eating.