In a medium saucepan, melt the butter over low heat. Add the cream, stirring. Increase the heat to medium and bring to a boil.
Place 6 ounces semisweet chocolate in a heatproof bowl. Add the cream mixture and let sit for 2 minutes. Stir the mixture until smooth. Cover with plastic wrap and refrigerate until firm, 1 hour.
Line a baking sheet with parchment paper. Using a small scoop or melon baller, scoop teaspoon-size balls of the chocolate mixture onto the pan; refrigerate for 30 minutes.
Meanwhile, in a double boiler, melt the white chocolate over simmering water, stirring until smooth; remove from the heat. (Keep the water simmering in case the chocolate hardens.) Using a fork, coat each truffle with the white chocolate. Set them on the baking sheet. Refrigerate to harden.
Rinse and dry the bowl, return to the simmering water and add the remaining 4 ounces semisweet chocolate to melt. Pour into a large, resealable plastic bag, make a tiny snip in one corner of the bag and pipe zigzags of dark chocolate over the truffles. Refrigerate overnight.