Line a baking sheet with aluminum foil and spray lightly with non-stick spray. Spread out your Moose Munch and add peanuts (if using).
In a medium pot over medium-low heat, add butter and brown sugar. Stir until butter is melted and brown sugar is dissolved.
Let the sugar mixture simmer for 6 to 8 minutes over low heat. If the mixture heats too quickly, it will break or even burn. Do not stir it during this phase or it will break as well.
Use a candy thermometer or instant read thermometer to test the doneness of the caramel. It is done when it registers between 290° F and 300° F.
When the caramel reaches the right temperature, remove it from the heat. Stir in vanilla extract, and then drizzle caramel over the Moose Munch. Be careful, as the caramel will be very hot. Quickly stir the caramel and Moose Munch together with a rubber spatula to coat and form clusters.
Transfer baking sheet to freezer to chill and harden. After 30 minutes, the caramel should be hard. Break the bark into pieces before serving.