On a lightly floured surface, roll the pie dough into a 12" circle.
Set the pie crust over a standard size pie plate and gently ease in to fit. (Don't press it down.)
Prick the bottom with a fork and trim the edge. Tuck overhang under itself and crimp the edges.
Chill for 30 minutes in the refrigerator.
Line crust with parchment paper and fill with dried beans or pie weights.
Blind bake until edges are lightly golden, approximately 18 to 20 minutes Remove the parchment and weights then continue baking for 10 more minutes, until golden and fully baked. Allow to cool completely.
In a medium saucepan, whisk the 1/2 cup sugar, gelatin, pumpkin pie spice, and salt.
In a medium bowl, whisk the pumpkin, egg yolks, vanilla and milk until smooth.
Add the pumpkin mixture to the pan and whisk to combine.
Place the pan over medium heat and cook until mixture is starting to steam, stirring constantly. When bubbles begin to form around the edges, remove from heat and transfer to a bowl. Cool to room temperature.
In a large bowl, lightly whisk the egg whites and cream of tartar.
Using a hand mixer, beat the egg whites until soft peaks form, approximately 2 minutes. Add remaining sugar a tablespoon at a time and beat again until stiff peaks form.
Transfer 1/4 of the whipped egg whites to the bowl with the cooled pumpkin mixture and gently fold to combine. Add the remaining whipped egg whites and fold to combine until no white streaks are showing.
Pour the pie filling into the pie shell and smooth the top with a spatula.
Refrigerate at least 4 hours or overnight before serving.
To serve, slice and top with whipped cream.
Pumpkin Chiffon Pie can be made up to 3 days in advance and stored in the refrigerator until ready to serve.