Preheat oven to 350 degrees F (175 degrees C). Grease a 9x5-inch loaf pan.
Beat cake mix, peanut butter, eggs, and 2 2/3 tablespoons softened butter together in a large bowl until you have a smooth batter. Spread about 2/3 of the batter into the prepared baking dish; press remainder into small balls and set aside.
Stir frosting, sweetened condensed milk, chocolate chips, and 2 teaspoons softened butter together in a bowl until smooth; spread atop the layer of batter in the baking dish. Lightly flatten balls of batter with your fingers and spread atop the frosting mixture.
Bake in preheated oven until golden brown, 20 to 25 minutes. Cool completely before cutting into 24 squares to serve.
