Put the chocolate chunks in a microwave-safe bowl. Melt the chocolate in the microwave for 30 seconds, then stir well. Continue to microwave in 30 second intervals until the chocolate is completely melted. This should take less than 2 minutes, depending on your microwave. Alternately, you can melt the chocolate in a double boiler over simmering water.
Pour the melted chocolate onto the lined baking sheet and use a spatula to spread it out. Top with the marshmallows and peanuts, adding as many as desired, and lightly pressing them into the chocolate.
Allow to cool at room temperature until set, around 1 1/2 hours.
Break the bark into pieces. Store the bark in an airtight container at room temperature.
The bark will stay fresh for several weeks in an airtight container at room temperature, making it perfect for gift giving.