Place 1 & 1/2 cups peanuts in the bottom of an ungreased 11″x 7″ pan (2-quart capacity).
Melt the butter and peanut butter chips in a large saucepan over low heat. Stir until thoroughly heated and smooth.
Remove the pan from the heat. Immediately stir in the condensed milk and marshmallows. Continue stirring until smooth and well-blended.
Pour the peanut butter mixture over the peanuts in the pan. Sprinkle the remaining 1 & 1/2 cups peanuts over the top of peanut butter mixture. Sprinkle lightly with coarse salt if you want the bars to be saltier.
Cover and refrigerate until chilled. (I recommend at least 4 hours.) Then cut into bars. Bars can be served chilled or at room temperature.
Notes
Store in an airtight container in the refrigerator up to a week.
These bars can be made in a 9″x 13″x 2" pan. Use another cup of peanuts, divided between the bottom of the pan and the top of the bars. The same amount of filling will work and give you slightly thinner bars.