Line a 9 x 9-inch baking pan with parchment paper and lightly grease it. Set aside.
Add 4 cups white chocolate chips to a large microwave-safe bowl and heat for 1-2 minutes on 50% power, stirring every 30 seconds until fully melted and smooth.
Stir 1 cup sweetened condensed milk, ¼ cup pumpkin puree, and 1 teaspoon pumpkin pie spice into the melted chocolate. Work quickly, as it will begin to thicken as soon as you begin stirring in.
Spread the fudge evenly into the prepared pan. Sprinkle the top with a few pinches of pumpkin pie spice, if desired. Cover with plastic wrap or aluminum foil and chill for at least 2 hours before cutting into squares and serving.
Notes
Fridge: Store leftover fudge in an airtight container in the refrigerator for up to 2 weeks. Serve chilled, or let it sit at room temperature for 30 minutes or so before serving.
Freezer: Freeze in a freezer-safe bag or container for up to 3 months. Let the fudge thaw overnight in the refrigerator, or at room temperature for about an hour before serving.