Line an 8×8-inch square baking pan with parchment paper and set aside.

Melt butter over low heat in a large saucepan. Use a saucepan with plenty of room because the mixture will rise as it boils.

Add brown sugar and evaporated milk. Increase heat to medium and cook, stirring constantly, until the mixture begins to boil.

Once boiling, attach a candy thermometer to the saucepan. Continue cooking and stirring constantly until the mixture reaches 238°F. Go by the thermometer rather than cooking time.

Remove from heat and let the mixture stand for 2 minutes.

Carefully pour the hot brown sugar mixture into a heat-safe mixing bowl and add vanilla extract. The mixture will be extremely hot, so use care when transferring it.

Add half of the powdered sugar and beat until combined. Add the remaining powdered sugar and beat for about 2 minutes, or until smooth.

Pour fudge into the prepared pan and spread evenly.

Let cool at room temperature for 1 hour, then refrigerate for 2 hours or until completely cooled and firm.

Lift the fudge from the pan using the parchment paper and cut into 36 pieces.