Preheat oven to 350°F. Line a 9x13” pan with foil and spray with cooking spray.
Cream butter, peanut butter, and both sugars in the bowl of an electric mixer fitted with the paddle attachment, about 2-3 minutes. Add egg, vanilla, milk, baking soda, and salt and mix until combined. Slowly add flour and mix until dough just comes together.
Press the cookie dough into the prepared pan.
Bake for 18-24 minutes, until it gets golden on top. Let cool completely before making fudge for the top.
Melt chocolate chips with sweetened condensed milk slowly over low heat, stirring often in a saucepan. Once it's smooth, remove from heat and immediately pour and spread over cooled bars. Sprinkle with sprinkles or chopped nuts, if desired. Cool completely before cutting into bars. Store in an airtight container for up to 3 days.
You can make the fudge portion in the microwave using a microwave safe bowl and stirring every minute until melted and smooth. It'll take awhile, be careful the bowl is hot!