Line a 10x15 baking dish with foil (or parchment paper). Spray the foil with cooking spray and set the dish aside.
In a dutch oven or large pot, melt the butter over medium heat. Reserve 4 cups of the marshmallows, and add the remaining marshmallows to the po with the butter
Continue cooking, stirring constantly, until the marshmallows are melted, about 5 minutes.
Remove the mixture from the heat and stir in the vanilla extract.
Stir in the chopped pretzels and chopped nuts.
Then gently stir in the rice chex, corn chex, and wheat chex.
Finally, stir in the reserved marshmallows.
Add the mixture to the prepared baking dish and gently press it down to form to the pan.
To decorate, press 24 pretzels evenly into the top of the treats.
Allow the marshmallow treats to cool at room temperature for 2 hours. Then place in the fridge for at least 1 hour to set up properly.
Slice into squares to serve.