Preheat the oven to 325 degrees . Toast the pecans on a baking sheet in the preheated oven for 8 minutes. remove the nuts from the oven and set aside to cool at room temperature until needed.

Heat 1-inch of water in the bottom half of a double boiler over medium heat. With the heat on, place the white chocolate in the top half of the double boiler. Use a rubber spatula to stir the chocolate until completely melted and smooth, about 4 minutes. Transfer the melted chocolate to a 1-quart bowl and set aside. Thoroughly wash and dry the top half of the double boiler.

Heat 1 inch of water in the bottom half of a double boiler over medium heat. With the heat on, place the semisweet chocolate in the top half of the double boiler. Use a rubber spatula to stir the chocolate until completely melted and smooth, about 6 minutes. Transfer the melted chocolate to a 4-quart bowl. Allow the chocolate to stand at room temperature for 5 minutes before proceeding. Add the pecans to the bowl of melted semisweet chocolate. Use a rubber spatula to fold in the pecans until combined. Pour the chocolate pecan mixture onto a nonstick baking sheet with sides and use a rubber spatula to spread evenly. drizzle the white chocolate, one tablespoon at a time, over the entire surface of the chocolate pecan mixture.

Use a rubber spatula to spread and blend the white chocolate into the surface of the chocolate pecan mixture creating a marbleized effect (be careful not to overblend which would diminish the marbleized effect). Allow the mixture to cool at room temperature for 30 minutes. Cover the baking sheet with plastic wrap and refrigerate until the bark is hard, about 1 hour. Remove the baking sheet from the refrigerator and transfer the bark to a cutting board. Use a cook's knife to cut the bark into irregular pieces. Refrigerate in a tightly sealed plastic container until ready to use.