Preheat the oven to 350º and generously butter a 7 x 10 in pan [or 8 x 8]

In a medium bowl, combine the melted butter and the sugar, stirring to mix well.

Stir in the cocoa.

Stir in the eggs, one at a time, and then the vanilla.

In a smaller bowl, whisk together the flour, baking and salt.

Stir the flour mixture into cocoa mixture, just until combined.

Layer about 1/2 of the brownie batter in the prepared pan.

Lay the peanut butter cup halves out evenly over the batter.

Carefully spoon the remaining batter over the top, and gently spread in a even layer taking care not to dislodge the peanut butter cups. A small offset spatula comes in handy for this.

Bake for about 25 minutes, until a toothpick inserted near the center comes out clean, except for a few moist crumbs. [make sure you aren't hitting a peanut butter cup with the toothpick when you test - if you are, try another spot] Move to a cooling rack.

Use a hand mixer to combine the butter and peanut butter.

Carefully beat in 1 cup of the sugar, along with 1 tablespoon of the milk.

Then, beat in the second cup of sugar, and the remaining milk, until very smooth and creamy. It should be of spreadable consistency - if not beat in another teaspoon or two of milk.

Cool in the pan for 2 hours before spreading on the frosting. Cut into bars. Store tightly covered for up to 4 days - after that, refrigerate.