Line a 9x9-inch pan with parchment paper or aluminum foil. Add 2 cups of peanuts and spread them out evenly over the bottom of the pan.
Melt the butter over medium heat in a medium-sized heavy bottomed saucepan. Add the marshmallows and stir until they're melted.
Reduce the heat to low and whisk in the dulce de leche and the peanut butter. When the mixture is completely combined, pour gently over the peanuts. Smooth it out with a rubber spatula then sprinkle the remaining peanuts evenly over the top. Use your hand to press them down gently into the caramel.
Refrigerate until set about 2 hours. Use the parchment to remove from the pan to a cutting board and cut into squares or bars with a sharp paring or chef's knife. Store tightly wrapped in the refrigerator or in a cool, dry place up to one week.