For the pie filling: Bring some water to a boil in a saucepan, then reduce the heat to a simmer. Find a heatproof mixing bowl that will fit over the top of the pan, but not sink it all the way in. (Do-it-yourself double boiler!)
Put the egg yolks in the mixing bowl. Add the granulated sugar and whisk until the yolks start to turn pale. Place the mixing bowl on top of the saucepan with the simmering water. Slowly add the bourbon and whisk to combine. Cook over the simmering water, using a rubber spatula to scrape the bowl, until thick, about 5 minutes. Remove from the heat, then cover with plastic wrap and refrigerate until cool, 20 to 25 minutes.
Place the mascarpone cheese and pumpkin puree in a bowl and stir until smooth. Combine the whipping cream, brown sugar, vanilla and cinnamon in a separate mixing bowl and whip until not quite stiff. Add the mascarpone and pumpkin mixture to the whipped cream mixture, followed by the chilled egg mixture. Fold gently until mixed well. Cover with plastic wrap and refrigerate for 1 hour.
For the pumpkin pie espresso syrup: Bring 1 1/2 cups water to a boil in a small saucepan. Remove the pan from the heat and slowly add the instant espresso, then stir until dissolved. Add the brown sugar, cinnamon and nutmeg and stir until the sugar has dissolved completely, 1 to 2 minutes. Set aside.
To assemble: Arrange the ladyfingers in a single layer in a 9-by-13-inch pan, breaking as necessary to fit. Spoon 2 teaspoons of the espresso syrup over each ladyfinger. Plop a third of the chilled mascarpone/pumpkin mixture on top and spread it into a thin layer. Grate over a thin layer of chocolate and sprinkle over 1/2 cup of the crushed gingersnaps.
Repeat the process 2 more times, finishing with the crushed gingersnaps. Cover and refrigerate for 3 hours or up to overnight before serving. This will allow for more moisture to soften the cookies and for the whole mixture to meld together. The tiramisu is best consumed within 24 to 36 hours after assembling, as the components will begin to break down and become soupy.