Line a 13- x 9-inch baking pan with parchment paper, allowing about 2 inches to extend over the long sides of pan.

Break bittersweet chocolate into small to medium chunks. Microwave chocolate in a medium-size microwavable bowl on MEDIUM (50% power) for 30 seconds, and stir. Continue to microwave until chocolate is completely smooth, shiny, and fluid, about 1 minute, stirring at 15-second intervals. To melt on the stove, bring 2 to 3 inches of water to a gentle boil in a saucepan over medium-high. Reduce heat to medium-low, and maintain a gentle simmer without steam. Place chocolate in a heatproof bowl, and set it over saucepan, making sure bowl does not touch water. Cook chocolate, stirring occasionally, until chocolate is melted, completely smooth, shiny, and fluid, 2 to 4 minutes.

Quickly pour melted bittersweet chocolate over bottom of prepared pan. Using an offset spatula or the back of a large spoon, spread chocolate into a fairly even layer, covering the bottom of the pan completely. Place in refrigerator, and chill until chocolate is cold and set, 30 minutes to 1 hour.

Break white chocolate bars into small to medium chunks, and repeat melting process. Quickly spread melted white chocolate over firm bittersweet chocolate layer. (Or leave a border around the edges, if you like.) Working quickly, sprinkle and scatter toppings in a single layer all over melted white chocolate, making sure every topping piece touches the melted chocolate so that it will adhere to white chocolate layer.

Cover and refrigerate until bark is firmly set, about 1 hour. To serve, use parchment paper handles to remove bark from pan. Use a sharp knife to cut bark into irregular chunks, or break into large pieces by hand. Store bark in an airtight container at room temperature for up to 7 days.