Line an 8- or 9-inch square baking pan with aluminum foil or parchment paper. If using foil, lightly coat with nonstick spray. Set aside.

Prepare all ingredients that will be added after boiling. Finely chop the white chocolate to promote quick and even melting. Remove the lid and foil seal from the marshmallow creme jar. Measure the vanilla extract and salt and set aside.

In a heavy 3-quart saucepan, melt the butter over medium-high heat. Before the butter has fully melted, use it to coat the sides of the saucepan nearly to the top. This step helps prevent sugar crystallization.

Add the evaporated milk and light corn syrup to the saucepan. Carefully add the brown sugar to the center of the pan, avoiding the sides.

Stir slowly with a wooden spoon until the butter is completely melted and all ingredients are fully incorporated.

Bring the mixture to a full rolling boil over medium-high heat, stirring constantly. A rolling boil is defined by bubbles rising from the center of the mixture. Once boiling, reduce the heat slightly to maintain a steady boil and continue stirring for exactly 5 minutes. If using a thermometer, the mixture should reach approximately 236°F (soft-ball stage).

Remove the saucepan from heat and immediately add the chopped white chocolate. Stir continuously until the chocolate is fully melted and the mixture is smooth.

Add the marshmallow creme and stir vigorously until fully incorporated. Work quickly, as the fudge will begin to thicken rapidly.

Stir in the vanilla extract and kosher salt. Continue mixing, scraping the bottom and sides of the pan, until the fudge is smooth, glossy, and an even light golden color.

Immediately pour the fudge into the prepared pan, spreading evenly if necessary.

Allow the fudge to cool at room temperature until fully set, approximately 3–5 hours. After about 30 minutes, cover the pan to prevent the surface from drying.

Once set, lift the fudge from the pan using the foil or parchment overhang. Slice into desired portions using a long, sharp knife.