Line an 8-inch square baking dish with parchment paper, leaving some overhang.

In a medium saucepan over medium heat, melt the unsalted butter.

Stir in the packed brown sugar, mixing well until combined.

Add the sweetened condensed milk and bring the mixture to a gentle boil, stirring constantly.

Allow the mixture to boil for about 5 minutes, continuing to stir.

Remove the saucepan from heat and stir in the vanilla extract.

Gradually add the powdered sugar, mixing until smooth and creamy.

Pour the fudge into the prepared baking dish and spread it evenly. Let it cool at room temperature until set, about 2 hours.