Preheat oven to 350 degrees F. Line an 11x7 (brownie pan) with parchment paper. Unwrap candy pieces.

In a small mixing bowl, whisk together the flour,baking powder, baking soda, and salt; set aside.

In a large mixing bowl, using an electric mixer, beat together the butter and peanut butter until smooth. Beat in the sugars, egg, and vanilla until well combined.

Add the flour to peanut butter mixture, alternating with the milk; one half of each at a time. Mix just until well combined.

Spread about two thirds of the cookie dough into the bottom of the pan. Arrange the candy pieces over the top of the dough. Drop remaining cookie dough, using a teaspoon, to cover as much of the candy as possible. Spread slightly with an offset spatula. Drizzle the sweetened condensed milk over the cookie dough.

Bake for about 28 to 30 minutes, or until cookie dough is golden brown and the dough seems almost set when slightly shaking the pan. Remove from the oven and allow to cool on a wire rack for about 2 hours.

Transfer cookie bars to a cutting board by lifting the parchment paper edges. Cut into bars of desired size.