Line a jelly roll pan with baking parchment.
Heat butter and cream in a small saucepan and bring slowly to a boil, stirring constantly.
Let it simmer for 1 minute, then remove from the heat.
Break the chocolate into pieces and add to the cream.
Stir until melted, then beat in the liqueur, if using.
Pour into the prepared pan and chill for about 2 hours until firm.
Break off pieces of mixture and roll them into balls.
Chill for a further 30 minutes before finishing the truffles.
To finish, melt the white chocolate.
Dip the balls in the chocolate, allowing the excess chocolate to drip back into the bowl.
Place on nonstick baking parchment and swirl the chocolate with a fork.
Let them harden.
Drizzle a little melted dark chocolate over the truffles if desired and let it set.
Place the truffles in paper cases to serve.
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