(1) (19.8 oz) package brownie mix
1 cup peanut butter chips
½ cup butter
(1) (14 oz) can sweeten condensed milk
Bake the brownies according to package directions. Let it cool in the pan. Do not cut.
Melt peanut butter chips and butter over low heat in a medium saucepan until smooth, stirring frequently. Remove from heat, and then stir in sweetened condensed milk. Spread evenly over the cooled brownies. Chill until set and cut into squares.
I kept the brownies at room temperature in an airtight jar, and they had been perfect for up to a week. When you want to make the peanut butter firmer and not so creamy, then place the brownies in the fridge. They even freeze well and hold in the freezer for up to 3 months. Place them overnight in the fridge to thaw before serving.
The texture of the brownies would be thick and gummy if underbaked (the toothpick will have batter stuck to it). When overbaked, the brownies will be dry and cakey (the toothpick will come out clean). Home ovens are fickle, and the calibration is bad. Use baking times as a reference and make the desired doneness based on the toothpick test.