Line a 9x9 baking dish with parchment paper. Set aside.
Remove 1 cup of marshmallows from the bag and set aside. Add the butter, marshmallows (minus 1 cup), vanilla extract, and peanut butter to a large saucepan over medium heat. Stir continuously until the ingredients melt together.
Remove from heat and pour in the Rice Krispies and remaining 1 cup of mini marshmallows. Stir the ingredients together until the Rice Krispies are coated.
Pour the ingredients into the prepared pan. Use your hands to firmly press down the krispie bars. If your hands are sticking to the bars, run them under cold water. This will prevent your hands from sticking to the bars.
Next, add the chocolate chips and heavy cream to a microwave-safe bowl. Microwave the ingredients for 30 seconds. Whisk together until the chocolate is melted and it's a thick, smooth texture. If need be, microwave another 10-20 seconds. Pour the chocolate sauce over the bars using an offset spatula to spread it evenly.
Sprinkle coarse salt on top of the bars (optional). Allow the bars to rest for 1 hour in the fridge or for 2 hours at room temperature (until the chocolate layer hardens). Then cut into squares and serve!