Grease a 9 by 13-inch low-rimmed baking sheet or casserole dish with the butter and set aside.
In the top of a double boiler or in a metal mixing bowl set over a pot of simmering water, melt the white chocolate, stirring occasionally with a rubber spatula. Use oven mitts or pot holders since the steam can burn.
Add the chopped toasted pecans and stir well.
Pour the melted white chocolate mixture into the buttered baking sheet and smooth the top with the rubber spatula.
Clean out the top of the double boiler or use a new clean bowl. Melt the semisweet chocolate in the top of the double boiler as above, stirring occasionally.
Spoon the melted semisweet chocolate on top of the melted white chocolate, then run the tip of a dull knife or the handle end of a teaspoon back and forth through the melted chocolate mixture to form a random "marbled" effect.
Transfer the baking sheet to the refrigerator until the chocolate hardens completely, about 1 hour.
Remove from the refrigerator and turn the baking sheet over onto a flat surface to remove the chocolate "bark." Using your hands, break the chocolate into whatever size pieces you like.
Serve immediately or store in an airtight container for up to 2 weeks.
YIELD: Thirty-six 2 x 1 1/2-inch pieces, serving 18 to 20 (lots!)