Prepare the Pan: Line an 8×8-inch baking dish with parchment paper, leaving some overhang for easy removal. Set aside.
Make the Fudge Base: In a large saucepan over low heat, melt the butter. Add the evaporated milk, granulated sugar, brown sugar, and 1 teaspoon salt. Stir until fully combined.
Boil the Mixture: Increase the heat to medium and bring the mixture to a rolling boil. Once boiling, do not stir. Continue cooking until the mixture reaches 234°F-237°F on a candy thermometer.
Cool the Mixture: Remove the saucepan from heat and let it cool undisturbed until the temperature drops to 110°F. Do not stir during this cooling period.
Prepare the Walnuts: While the mixture cools, toast the walnuts. In a small skillet over medium heat, add the vegetable oil, chopped walnuts, and the remaining 1 teaspoon of salt. Cook for 3-5 minutes, stirring frequently, until the walnuts are fragrant. Remove from heat and set aside.
Melt the White Chocolate: In a microwave-safe bowl, heat the white chocolate in 15-second intervals, stirring between each, until smooth and creamy.
Combine Ingredients: When the fudge mixture cools to 110°F, add the melted white chocolate, marshmallow creme, maple extract, and ½ cup of the toasted walnuts. Stir until well combined.
Set the Fudge: Pour the fudge mixture into the prepared baking dish and spread it evenly. Sprinkle the remaining toasted walnuts on top. Let the fudge cool at room temperature for 2-3 hours, or until firm.
Serve: Once the fudge is set, lift it out of the pan using the parchment paper overhang. Cut into squares and enjoy!