Stir together the sugar, butter, milk, pumpkin, corn syrup, and pumpkin pie spice in a large saucepan over medium-high heat. Cook, stirring constantly, until mixture comes to a boil. Continue cooking, stirring constantly, until a candy thermometer reads 234 degrees F (soft-ball stage).
Remove pan from heat. Stir in white chocolate, marshmallow fluff, and vanilla until well blended. Pour into a greased, aluminum foil or parchment paper-lined pan. Let stand 2 hours or until completely cool. Cut fudge into squares.