Add your butter to a large saucepan over medium-low heat and let it melt completely, swirling the pan occasionally. Keep the heat gentle here, since butter that gets too hot can turn the marshmallows grainy later. This usually takes about two minutes.
Once the butter is melted, add in your mini marshmallows along with the salt. Stir continuously until they're fully melted and smooth, which takes roughly three to four minutes. Remove the pan from the heat as soon as it's smooth.
Stir the vanilla extract into the warm marshmallow mixture first, then fold in the crispy rice cereal a little at a time. Mix gently so you don't crush the cereal, just enough to coat every piece evenly with marshmallow.
Add your reserved cup of large marshmallows and fold them through quickly before the mixture cools too much. This step is what gives the finished treats those extra gooey, stretchy pockets people love.
Transfer the mixture into your parchment-lined pan and press it down gently with a lightly greased spatula. Avoid pressing too hard, since packing it in tightly is what makes rice krispie treats turn out dense and hard.
Let the pan sit at room temperature for about 30 minutes until fully set. Once cooled, lift the whole slab out using the parchment paper and slice it into even squares with a sharp knife