Line an 8-inch square pan with aluminum foil or parchment paper and spray with non-stick cooking spray. Set aside.
In a large microwaveable bowl, melt peanut butter chips, 1 cup of the sweetened condensed milk and 2 tablespoons butter, stir occasionally in the microwave on high for 1 minute. Stir until smooth, then stir in peanuts.
Spread mixture into prepared pan. Set aside.
In another large microwaveable bowl, melt chocolate chips, remaining sweetened condensed milk and butter in the microwave on high for 1 minute. Stir until smooth.
Immediately spread chocolate mixture on top of peanut butter mixture. Chill 2 hours or until firm. Turn fudge onto cutting board; peel off paper and cut into 36 squares. Store covered.