Peanut Butter Polka Dot Bars

Preheat oven to 350 degrees F.
Line a 9 x 13 pan with a large sheet of foil, then spray the foil with nonstick spray.
Meanwhile, in a large mixing bowl, beat butter, peanut butter, and brown sugar until well blended. Add eggs and vanilla and beat again. In a separate bowl, combine oats, flour, baking soda, and salt; add to peanut butter mixture and combine well. Press all but 2 cups of dough into the foil-lined 9 x 13 pan.
Meanwhile, prepare chocolate filling by melting the butter in a saucepan over medium low heat; stir in the cocoa, sugar, and condensed milk. Cook, whisking constantly until smooth and thickened; remove from heat and whisk in vanilla extract.
Spread the warm prepared chocolate filling over the cookie dough that's been pressed into the 9 x 13 pan. Now, using your fingers, crumble the reserved cookie dough evenly over the top of the chocolate filling. (You'll see some of the filling peeking through the crumble.) Sprinkle the Reese's Pieces over the crumble topping.
Bake, uncovered, on the center rack of the oven for 25-30 minutes or until the top is golden brown and bars seems set at the center.
Cool the bars completely, then carefully lift the foil and sheet of bar cookies out of the pan, placing it onto a cutting board. Using a knife or a bench scraper, cut the sheet into the desired size of bar cookies, then store them in an air-tight container.