Peanut Butter Chocolate Bars

For the Crust:
1 package yellow cake mix
1 1/4 c. creamy peanut butter
8 T. butter, melted
2 large eggs

For the Filling:
2 c. (12 oz.) chocolate chips
1 can (14 oz.) sweetened condensed milk
2 t. vanilla

1. Place a rack in the center of the oven and preheat the oven to 350 degrees. Set aside an ungreased 13- by 9-inch metal pan.
2. For the crust, place the cake mix, peanut butter, 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 batter will be thick. Reserve 1½ cups crust mixture for the topping. Turn the remaining batter into the pan and spread it evenly with your fingertips so that it reaches slightly up the sides of the pan. Set the pan aside.
3. For the filling, place the chocolate chips and condensed milk in a large microwave-safe bowl in the microwave oven on high power for 1 minute. Remove the bowl and stir until the chocolate chips melt. Stir in the vanilla. Pour the chocolate mixture over reserved crust, and spread so that it reaches the edges of the pan. With your fingertips, crumble the reserved crust mixture over the top of the filling. Place the pan in the oven.
4. Bake the bars until they are golden brown, 30 to 35 minutes. Remove the pan from the oven and cool 30 minutes. Cut into 24 to 30 bars and serve.