In a large, heavy saucepan, whisk together brown sugar, cornstarch and half-and-half. Place over medium-high heat and stir until sugar is just dissolved, about 2 min. Whisk in egg yolks, pumpkin pie spice, salt, molasses and pumpkin purée. Reduce heat to medium and cook, stirring constantly to prevent clumping, until pudding thickens and just boils, 10 to 15 min. Remove from heat. Place pan in a bowl of ice and water and stir occasionally until custard cools.
Remove from ice bath, dry pan bottom and transfer custard to prepared crust. Smooth surface and cover with plastic wrap, gently pressing wrap onto surface of pudding; refrigerate 3 hr or overnight.
In another heavy saucepan, stir together granulated sugar and 1/4 cup water over medium-high heat until sugar is completely dissolved. Remove spoon and brush inside edge of pan with a wet pastry brush to remove remaining sugar crystals, then swirl pan occasionally until sugar begins to caramelize. When caramel is medium amber in color, pour onto parchment in a thin layer and sprinkle with pumpkin seeds. Let cool completely and break into shards.
Whip cream and confectioners' sugar until firm peaks form and spread onto chilled pie. Garnish with pumpkin-seed brittle.