Prepare a 8×8-inch baking dish by coating the bottom with 1/2 Tbsp butter and lining it with parchment paper. The butter will help keep the parchment in place.
Add the salted butter into a large stainless steal or enamel coated dutch oven pot over medium heat, and let it fully melt. Use a wooden spoon to stir the butter constantly until it turns a dark amber color. Avoid using nonstick or really any dark colored pot since you will need a light colored pot to be able to keep a close eye on the color of the butter.
Add 8 cups of marshmallows, frequently stirring until melted. Once melted, stir in the vanilla and remove from heat.
Fold in the Rice Krispies cereal and stir until they are fully coated in melted marshmallows. Add the remaining 2 cups of marshmallows. Stir until the marshmallows are well-coated.
Transfer the mixture into your prepared baking dish. Press it into an even layer. I like to spray a spatula with non-stick spray to help compress the bars into the pan. Let the the bars cool to room temperature before cutting and serving.
Notes
Butter your baking pan before adding parchment paper- Doing this will keep the parchment paper in place so it doesn't shift when you add the Rice Krispie and marshmallow mixture.
Take the marshmallows off the heat once melted- Be careful not to burn the marshmallows and butter. Take them off the heat while you combine them with the Rice Krispies.
Add marshmallows at the end- This step adds tons of gooey marshmallow chunks in the middle of these treats that make them irresistible.
Keep the mixture from sticking to your spatula- To keep it from sticking, spray a spatula with non-stick spray before compressing the bars into the pan.