1. Heat oven to 325 degrees. In a food processor, combine graham crackers, 1/3 cup plus 2 tablespoons granulated sugar, cocoa powder, 1/4 teaspoon cinnamon, and pinch nutmeg; process until finely ground. Transfer to a medium bowl; mix in butter with a wooden spoon. Press mixture onto bottom and sides of a 10-inch tart pan. Bake until set, 12 to 15 minutes. Set crust aside to cool.

2. In a small bowl, place brandy and 2 tablespoons water. Sprinkle gelatin powder over liquid; set aside to soften, 10 minutes. In the bowl of an electric mixer fitted with the whisk attachment, beat eggs on medium-low speed until fluffy. While the mixer is running, combine remaining 3/4 cup sugar and 1/4 cup water in a small saucepan over medium-high heat. Cook, stirring occasionally, until temperature registers 245 degrees (firm-ball stage) on a candy thermometer, about 5 minutes. Immediately turn mixer to high speed. Carefully drizzle sugar mixture into eggs in a thin stream; continue to beat until mixture increases in volume and is pale yellow, about 5 minutes. Meanwhile, place bowl with softened gelatin over a saucepan of simmering water; stir until gelatin has dissolved.

3. Turn mixer to low; add gelatin mixture to egg mixture. Add pumpkin puree, remaining 1/2 teaspoon cinnamon and 1/4 teaspoon nutmeg, the allspice, ginger, salt, and sour cream; combine. Pour filling into pie crust; refrigerate until set, 4 hours or overnight.

4. To serve, whisk heavy cream, confectioners' sugar, and vanilla in a medium bowl until stiff. Using an Ateco #864 star tip, pipe whipped cream in rosettes onto pie (or spread the whipped cream evenly over the top of the pie with a spatula). Sift remaining tablespoon cinnamon on top. Serve chilled.