Line an 8-inch or 9-inch baking pan with aluminum foil. Set aside.
In a medium saucepan, melt butter over medium heat. Stir in brown sugar and milk. Once the mixture comes to a full boil, set a timer for 2 minutes. Boil and stir once or twice to scrape the sides of the pan, then remove from heat. Immediately add peanut butter and vanilla (if using). Stir until smooth and let mixture cool slightly (about 5 to 10 minutes)
Place confectioners’ sugar in a mixing bowl and pour peanut butter mixture over the sugar. Use an electric mixer to beat until smooth.
Pour mixture into prepared pan and use rubber spatula to press down firmly and evenly. Cover with plastic wrap and chill for at least 4 hours or until firm. Slice and serve.
Notes
Use a creamy no-stir peanut butter. If using natural, be sure the peanut butter is not oily.Optional mix-ins: Stir in 1/2 cup chocolate chips or nuts before transferring the mixture to the pan.Store peanut butter fudge in an airtight container at room temperature for up to 2 weeks.