Prepare your baking pan by lining it with parchment paper, leaving some overhang for easy removal of the fudge later.
In a medium saucepan over medium heat, melt the unsalted butter.
Once melted, add the brown sugar and sweetened condensed milk to the butter. Stir continuously until the mixture is well combined and smooth.
Bring the mixture to a gentle boil. Continue to cook for about 5 minutes, stirring constantly to prevent burning. If you have a candy thermometer, the mixture should reach 234°F (soft-ball stage).
Remove the saucepan from heat and stir in the vanilla extract.
Gradually add the powdered sugar to the mixture, stirring until well blended. Be sure there are no lumps.
Fold in the chopped pecans until evenly distributed throughout the fudge.
Pour the fudge mixture into the prepared baking pan and spread it evenly.
Allow the fudge to cool at room temperature for about 30 minutes, then transfer it to the refrigerator to set for at least 2 hours.
Once fully set, lift the fudge out of the pan using the parchment overhang and cut it into 24 squares.