Line a square baking dish with parchment paper. Lightly butter the paper and set aside.
Place the ¼ cup of butter in a large microwave safe bowl. Melt the butter in the microwave, about 1 to 2 minutes.
Add the marshmallows to the melted butter and stir well to coat.
Place the bowl back in the microwave and heat the marshmallows until they are puffy and inflated, about 1 to 2 minutes.
Remove the bowl from the microwave and, using a wooden spoon or rubber spatula, stir the marshmallows.
If they are still holding their shape after stirring, continue to cook and stir them in 30 second increments until they are smooth and creamy.
Add the vanilla to the melted marshmallows and stir well to combine.
Pour the Rice Krispies into the bowl. Stir well until all of the Rice Krispies are coated in melted marshmallow and the marshmallow is evenly distributed.
Scrape the mixture into the prepared buttered pan.
Use a small piece of parchment paper to gently press the Rice Krispies evenly into the pan, including the corners.
Cool the Rice Krispie treats for at least 30 minutes before slicing.