Preheat oven to 350 degrees and line a 9X13-inch baking dish with aluminum foil. Coat the foil lightly with cooking spray.
In a large bowl, combine melted butter, brown sugar, baking soda, and salt. Use a hand-held mixer to mix everything together.
Add flour and oats and mix until blended well.
Set aside 1 heaping cup of the oat mixture. Press the remaining into the bottom of the prepared pan. Bake for 10 minutes.
Combine sweetened condensed milk and peanut butter in a bowl and mix until blended. Pour mixture over prepared crust, using a spatula to even it out.
Sprinkle half of the reserved oat mixture over the peanut butter layer. Then sprinkle the Reese's Pieces over the top.
Top with the remaining oat mixture. Bake for about 25 minutes. Let cool before cutting into squares. Store at room temperature in an air tight container or ziptop bag.