In a large saucepan, melt the butter over low heat. Add the marshmallows; stir until melted, 5 minutes. Stir in 1/3 cup peanut butter and 1/2 tsp. salt. Remove from the heat; stir in the cereal.
Cover the inside of a medium (3 qt.) bowl with plastic wrap, leaving a few inches of overhang. Coat the plastic and your fingers with cooking spray. Press the cereal mixture into the bowl. Refrigerate for 30 minutes.
In a double boiler, stir the chocolate over medium-high heat until melted, 5 minutes.
Pour the chocolate into the cereal bowl; spread with a rubber spatula to coat the inside. Refrigerate for 30 minutes.
In a bowl, whisk the remaining 1 cup peanut butter, the brown sugar and remaining 1/4 tsp. salt. Whisk in 1/2 cup cream.
Using an electric mixer fitted with the whisk attachment, beat the remaining 1 1/2 cups cream on medium speed until thick, about 1 minute. Slowly add the granulated sugar and vanilla; beat to soft peaks, 2 minutes. Fold into the peanut butter mixture in 3 additions.
Pull up the plastic to transfer the cereal bowl to a platter; discard the plastic. Spoon in the peanut butter cream. Top with the peanuts. Have guests spoon cream onto plates and break off pieces of the bowl.
The Do's and Don'ts of Cooking with Chocolate