1. To make the pie: Preheat the oven to 350°F. Bake the crust until warm and fragrant, 12 to 15 minutes. Set aside to cool.
2. Sprinkle the gelatin over 3 tablespoons of water in a small cup and let sit for 5 minutes. Meanwhile, combine the pumpkin puree, milk, sugar, egg yolks, spice, and salt in a medium saucepan. Cook over medium-low heat, stirring with a wooden spoon, until the mixture begins to bubble and thicken, 6 to 8 minutes.
3. Whisk the gelatin mixture into the saucepan with pumpkin mixture until combined and the gelatin is dissolved completely. Let sit at room temperature, stirring frequently, until it is cool but not set, 30 to 40 minutes. Transfer to a large bowl.
4. Beat ¾ cup of the cream in a large bowl until semi-stiff peaks form. Fold the whipped cream into the cooled pumpkin mixture, folding just until combined. Pour into the graham cracker crust. Place plastic wrap directly on the filling and refrigerate until firm, about 3 hours or overnight.
5. For the brittle: Spray a rimmed baking sheet with cooking spray. Have a candy thermometer, a bowl of water and a pastry brush ready.
6. Put the sugar and ¼ cup water in a medium saucepan and place over medium-high heat. Cook without stirring, allowing the water to evaporate as the sugar begins to melt. Dip the pastry brush in the bowl of water and brush down the sides of the pan if any sugar crystals begin to form around the edges. When the sugar turns a deep caramel color and registers 300°F. on the candy thermometer, about 15 minutes, pick up pan and gently swirl to melt any sugar that hasn't dissolved. Carefully pour the melted sugar onto the prepared baking sheet, spreading it out as much as possible. Immediately sprinkle the toasted pecan pieces and salt evenly over the top and allow to cool completely, about 30 minutes.
7. Beat the remaining ½ cup cream in a until stiff peaks form. Spread the cream over the top of the pie. Break up the brittle into shards and put them into the whipped cream; slice the pie and serve with more brittle.
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