Melt the butter in a medium saucepan over medium heat.
Add the brown sugar and milk, stirring constantly. Bring to a boil and let it cook for 2 minutes, ensuring the sugar dissolves completely.
Remove from heat and stir in peanut butter and vanilla extract until smooth and creamy.
Gradually mix in the powdered sugar, stirring until fully combined. The mixture should be thick and smooth.
Pour into a lined 8×8-inch baking dish, spreading evenly. Let it cool at room temperature for at least 2 hours before cutting into squares.
Video
Notes
For variations, try adding melted chocolate for a swirled effect, using chunky peanut butter for extra texture, or sprinkling sea salt on top for a sweet-salty balance.