Makes one 8-inch (20 cm) square pan.
Have a parchment-lined baking tray ready. Place the water and lemon juice into a medium saucepan and add the sugar. Bring this up to a boil over high heat without stirring, and occasionally brushing down the sides of the pot with water. Once the sugar turns an amber colour, remove the pan from the heat, add the pecans all at once and stir to coat. Spoon the pecans out onto the baking tray and spread into a single layer. Let this cool while preapring the crust and filling.
For the crust, lightly grease and line an 8-inch (20 cm) square pan with parchment paper so that the paper comes up the sides. Stir the graham crumbs with the cinnamon and add the melted butter, stirring until the crumbs are coated. Press this into the preapred pan and chill while preapring the chiffon layer.
Measure the milk into a medium saucepan and whisk in the gelatin, letting this sit for a minute. Add the sugar, cinnamon and ginger and whisk over medium-low heat until you just begin to see a bit of steam rise from the milk, about 3 minutes. (If using agar agar powder, you need to bring the milk to a full simmer and whisk for one minute to fully activate it.) Remove the pan from the heat and whisk in the pumpkin. Transfer the mixture to a bowl and set aside to cool to room temperature.
Whip the cream until it holds a soft peak when the beaters are lifted. Fold this into the cooled pumpkin mixture in two additions, and spread in an even layer over the chilled crust. Chill this for at least 2 hours, uncovered.
Shortly before serving, chop the pecan pralines and sprinkle overtop before slicing to serve.