Place rack in the center of the oven and preheat the oven to 325 degrees F. Set aside an ungreased 13x9 inch baking pan.
Place cake mix, peanut butter, melted butter and eggs in a large mixing bowl. Blend with an electric mixer on low speed for 1 minute. Stop the machine and scrape down the sides of the bowl with a rubber spatula. The mixture will be thick. Reserve 1 1/2 cups for the topping. Transfer the remaining crust mixture to the pan. Using your finger tips, press the crust evenly over the bottom of the pan so that it reaches all sides. Set aside.
For the filling, place the chocolate chips, condensed milk, and 2 tablespoons butter in a medium-size heavy saucepan over low heat. Stir and cook until the chocolate is melted and the mixture is well combined, 3-4 minutes. Remove the pan from the heat and stir in the vanilla. Pour the chocolate mixture over the crust, and spread it evenly so that it reacheds the sides of the pan. Crumble the reserved crust and scatter it evenly over the chocolate. Place in the oven.
Bake until light brown, 25 to 30 minutes. Remove from the oven and place on a wire rack to cool for at least 30 minutes before cutting.
Cut into 48 bars. Store in an airtight container at room temperature for up to 3 days or in the refrigerator up to one week. (If they last that long) These freeze beautifully.