How to Make It
Preheat oven to 200°F. Line a quarter-size (13- x 9- x 1-inch) rimmed baking sheet with parchment paper, letting parchment extend 2 inches past long sides.
Spread chopped semisweet chocolate in an even layer in prepared baking sheet. Sprinkle chopped white chocolate evenly over semisweet chocolate. Turn off oven. Place baking sheet in oven; bake until chocolate melts, about 10 minutes.
Remove baking sheet from oven, and swirl mixture with a wooden pick or the tip of the knife to create a marbled surface. Sprinkle peanuts and pretzels evenly over melted chocolate. Chill until solid, about 30 minutes, or let stand at room temperature until solid, about 1 hour.
Using excess parchment as handles, remove bark from baking sheet. Break bark into bite-size pieces.