Using a pencil, draw an 8 x 11-inch rectangle on a piece of parchment paper. Turn the parchment paper over so the pencil mark doesn't get onto the chocolate and place it on a sheet pan.
Place three-quarters of the bittersweet chocolate in a heatproof glass bowl and put it in the microwave on high for 30 seconds. (Time it carefully.) Stir the chocolate with a rubber spatula, return it to the microwave for another 30 seconds, then stir again. Continue to heat and stir in 30-second intervals until the chocolate is just melted. Immediately stir in the remaining bittersweet chocolate and allow it to sit at room temperature, stirring often, until it's completely smooth. (If you need to heat it a little more, place it in the microwave for another 15 seconds.)
Pour the melted chocolate onto the parchment paper and spread it lightly to fill the drawn rectangle. Sprinkle the top evenly with the peanuts, chocolate chips and marshmallows. Press them lightly so they will set in the chocolate.
Place the white chocolate in a small, heatproof glass bowl and microwave it for 30 seconds. Stir with a rubber spatula and return to the microwave for 15 seconds, then stir again. Continue to heat and stir in 15-second intervals until the chocolate is just melted.
Using a spoon, drizzle the white chocolate onto the bark in straight lines.
Set aside for at least 3 hours until very firm. Cut or break the bark in 16 pieces and serve at room temperature.