Line an 8x8-inch square baking pan with parchment paper, letting some overhang on two sides to create handles. Lightly grease the parchment with a bit of neutral oil or butter so the fudge releases cleanly.
1 8x8-inch square baking pan, parchment paper, neutral oil spray or butter
In a medium, heavy-bottomed saucepan, combine the white chocolate chips, sweetened condensed milk, and butter. Set over low heat and stir constantly with a silicone spatula or wooden spoon until the mixture is smooth, glossy, and the chips are fully melted. Do not increase the heat; keep it low to prevent the chocolate from seizing.
2 1/2 cups white chocolate chips, 14 oz can sweetened condensed milk, 4 tablespoons unsalted butter
Once the mixture is smooth, add the pumpkin puree, pumpkin pie spice, vanilla extract, and fine sea salt. Stir well, scraping the sides and bottom of the pan, until everything is fully combined and the color is an even soft orange.
1/3 cup pumpkin puree, 2 teaspoons pumpkin pie spice, 1 teaspoon vanilla extract, 1/4 teaspoon fine sea salt
Keep the mixture over low heat for another 2–3 minutes, stirring constantly. It will thicken slightly and look thick and satiny. As soon as you see it start to bubble lightly around the edges, remove the pan from the heat. Do not let it reach a rolling boil.
Carefully taste a small cooled spoonful of the mixture to check the spice level. If you prefer a stronger spiced flavor, stir in up to an additional 1/4 teaspoon pumpkin pie spice off the heat. Remember, the flavors will continue to bloom as the fudge chills.
2 teaspoons pumpkin pie spice
Immediately pour the hot fudge mixture into the prepared pan. Use an offset spatula or the back of a spoon to spread it evenly into the corners. Tap the pan gently on the counter a few times to pop any air bubbles.
1 8x8-inch square baking pan
While the fudge is still warm and soft, sprinkle your choice of toppings—chopped pecans or walnuts, crushed gingersnaps or graham crackers, a light sprinkle of flaky sea salt, or a dusting of cinnamon or pumpkin pie spice—over the surface. Press very lightly so they adhere without being pushed deep into the fudge.
1 optional toppings, 1/4 cup pecans or walnuts, 2 tablespoons crushed gingersnaps or graham crackers, flaky sea salt, ground cinnamon or pumpkin pie spice
Cover the pan loosely with plastic wrap or foil, avoiding direct contact with the surface of the fudge if possible. Refrigerate for at least 3–4 hours, or until very firm. For best flavor and texture, chill overnight.
Use the parchment handles to lift the chilled fudge out of the pan and onto a cutting board. With a sharp knife, cut into approximately 1-inch squares. If the knife sticks, rinse it in warm water, dry thoroughly, and continue slicing.
Serve the pumpkin spice fudge chilled or at cool room temperature. The fudge is rich, so small squares go a long way.