Preheat the oven to 350 degrees. Spray a 9 inch square baking dish with nonstick cooking spray.
In a medium saucepan, melt the butter and brown sugar until the sugar dissolves. Let cool 10 minutes.
Add the eggs and beat into the mixture.
Stir in the vanilla.
Fold in the flour and salt.
Once completely cool, fold in the peanut butter cups.
Spread evenly in the prepared dish.
Bake 25-30 minutes. Don't overbake. Transfer to a wire rack and allow to cool. Cut into bars and serve.