Prepare a 9 x 13-inch baking dish with parchment paper or cooking spray.
Melt 4 tablespoons of unsalted butter in a large saucepan over medium heat. Once the butter melts, reduce heat to low, add (1) 13-ounce jar of marshmallow fluff, and stir until the mixture is smooth and creamy. (About 2-3.)
Remove the mixture from the heat, and stir in 1 teaspoon of pure vanilla extract.
Gradually add 7 cups of Rice Krispie cereal to the marshmallow mixture, stirring until all of the cereal is evenly coated with marshmallow.
Press the mixture into the prepared baking dish using a piece of wax paper. Let the Rice Krispie treats cool completely before cutting them into squares. (Do note: This can be as much as 2-3 hours or more, as the fluff doesn't "set" the treats as quickly as traditional rice krispie treats.)
Notes
Use Fresh Marshmallow Fluff: Older fluff can become hard, or really stiff, so this would make stiff, hard, rice krispie treats.
Control the Stickiness: Use wax paper to press the treats down in to the dish, or lightly wet hands with water before pressing down the treats to prevent the mixture from sticking to your hands.
Allow to Fully Cool: Allowing the treats to fully cool will help make sure that the treats don't fall apart - and this could be a while. Make sure you allow enough time for the treats to set. I find that this takes close to 3 hours, since the fluff takes longer to set than marshmallows.