Make the crust. Set an oven rack in the lower third and preheat to 375°F. Stir together the graham cracker crumbs, brown sugar, cinnamon, and salt. Add the melted butter and mix until the crumbs are evenly moistened. Press firmly into the bottom and up the sides of a 9-inch glass pie dish.
Prebake the crust. Bake for 10 minutes, then remove the crust and let it cool. Keep the oven at 375°F.
Mix the filling. In a medium bowl, whisk together the granulated sugar and flour, then stir in the brown sugar. Add the milk, heavy cream, vanilla, and salt. Whisk gently just until combined. Gently whisk in the pumpkin purée and pumpkin pie spice until evenly blended.
Bake the pie. Pour the filling into the prebaked crust. Bake for 28–32 minutes, or until the edges look set and the center still has a slight jiggle. In a deeper pan, it may take 45–60 minutes. If the crust edges are browning too quickly, loosely cover them with foil.
Cool and chill. Let the pie cool to room temperature, then refrigerate for at least 2 hours before slicing. Serve each slice with a generous scoop of vanilla bean ice cream (highly recommended!).