Place the cream in a heat-proof bowl, and set the bowl over a pan of simmering water. Cook until heated through. Using a wire whisk, slowly stir the white chocolate into the warm cream until completely melted. Whisk in the liqueur and vanilla. Cover and chill for 45 minutes to an hour until pliable but firm enough to scoop.
Preheat the oven to 350 degrees F.
With two teaspoons or a 1 1/4-inch ice cream scoop, make dollops of the chocolate mixture and place on a baking sheet lined with parchment paper. Refrigerate for about 15 minutes, until firm enough to roll into rough spheres.
Place the chopped hazelnuts on a sheet pan and place them in the oven for 8 minutes. Cool to room temperature.
Melt the bittersweet chocolate in a heat-proof bowl, set over a pan of simmering water.
Drizzle the melted bittersweet chocolate over 10 of the truffles. Roll the remaining truffles in the chopped nuts. Chill until ready to serve.
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