Spray a 9x11 baking pan with nonstick cooking spray and set aside.
In a large pot (it must be large!) melt ¾ cup salted butter over medium heat. Next, add almost all of the 32 oz mini marshmallows (about 2 bags), but leave 2 cups worth out (save those for later).
Reduce the heat to medium-low and gently stir until the marshmallows melt, about 3-4 minutes. Do not leave the stove during this time as the ingredients can easily burn.
Remove from heat and then stir in 1 tablespoon vanilla extract.
Now add 12 cups of rice krispies cereal, and 2 cups of mini marshmallows to the pot (this step is why you need a big pot for this recipe). Gently mix everything together so that the rice krispies are evenly coated with the melted marshmallows.
Scoop the rice krispies mixture into the prepared baking pan. Lightly spray a silicone spatula with nonstick cooking spray and use it to help spread out the mixture into an even layer.
NOTE: If you want softer rice krispies, then press them lightly into the pan. If you want thick dense rice krispies treats (which is my personal favorite) then press the mixture firmly down into the pan.
Cover the pan with plastic wrap and let it sit on the counter or in the fridge for at least an hour. I prefer to let them chill overnight in the fridge to get clean crisp lines when I slice them.
Now slice into 12 or 24 servings (depending on how big you want them), serve and enjoy!