Line a 9x13 pan with parchment paper or lightly grease it.
In a large pot, melt the butter over low heat.
Add the marshmallows and stir slowly until melted.
Do not rush this part. High heat makes hard treats later.
Once smooth, remove from heat and stir in vanilla and a pinch of salt.
Pour in the cereals and gently fold until coated.
Tip: reserve about 1/2 cup cereal and mix it in last so you keep some fully intact marshmallows for texture and color.
Transfer mixture to the prepared pan.
Lightly butter your hands or the back of a spatula and gently press down.
Do not pack tightly — soft pressing = soft treats.
Let sit about 30–40 minutes before slicing.
For clean cuts, use a sharp knife lightly greased with butter.