Preheat the oven to 350 degrees F. Transfer the pie dough to a 9-inch pie plate and crimp the edges. Prick the bottom of the crust all over with a fork and line with foil and pie weights or dried beans. Bake until golden around the edges, about 20 minutes. Remove the foil and weights and bake until the bottom is dry, 10 to 15 minutes. Transfer to a rack to cool completely.
Meanwhile, make the filling: Whisk the half-and-half, 2/3 cup sugar, the eggs and pie spice in a medium bowl. Whisk in the pumpkin. Pour the filling into the crust.
Bake the pie until the center is just slightly jiggly but not loose, about 1 hour. Transfer to the rack to cool completely.
Meanwhile, make the topping: Line a baking sheet with foil and mist with cooking spray. Cook the remaining 1/2 cup sugar and the water in a small saucepan over medium-high heat, swirling the pan a few times, until the mixture is light brown, about 5 minutes. (Brush down any sugar crystals on the sides of the pan with a wet pastry brush.) Stir in the pepitas (the mixture will be thick and sticky). Stir in the salt and cook, stirring, until the mixture thins and turns dark brown, 1 to 2 minutes. Pour onto the prepared baking sheet and smooth out any clumps. Let the brittle cool completely.
Beat the heavy cream in a medium bowl with a mixer on medium speed until medium peaks form, 2 to 4 minutes. Top the pie with the whipped cream; break up the brittle and sprinkle on top.