In a large heatproof bowl (preferably with a pour spout), whisk the egg yolks and cornstarch together with a fork. It may not look like it will all come together (it will be dry at first), but keep mixing until the mixture is thick and combined. If it’s not coming together at all, add a few drops of the milk you need in step 2 to bring it together. Set aside.
Combine the milk and sugar in a medium saucepan over medium heat. Whisk until the sugar has dissolved, then allow to come to a simmer. If you have an instant-read thermometer, the temperature should reach 210°F (99°C). Remove from heat. Pour the warm milk and sugar mixture in a slow and steady stream into the egg yolk and cornstarch mixture, whisking the entire time. Keep those egg yolks moving so they don’t scramble. Once that’s all combined, add the pumpkin and whisk to combine. Pour the mixture back into the saucepan through a sieve, to strain out any egg yolk solids that may have formed during tempering. Use a spatula or the back of a spoon to gently press any remaining pumpkin through the sieve. Return the saucepan to medium heat, and whisk constantly until the mixture has thickened into a pudding-like consistency. This usually takes about 5 minutes. (For a more accurate test, it’s done when the temperature reaches 185–190°F (85–88°C).) Remove from heat. Whisk in butter, vanilla, pumpkin pie spice, and a pinch of salt.
Transfer the pumpkin pastry cream to a heatproof bowl and place a piece of plastic wrap or parchment directly on the surface of the pastry cream, to prevent a skin from forming. Allow to cool at room temperature for 10 minutes, then place the bowl in the refrigerator. Refrigerate for at least 3 hours, and up to 2 days. Makes about 3 cups, around 720g. After chilling, the pumpkin pastry cream is quite thick; whisk or stir it to smooth out again before using in step 7. Place the ginger snaps/Biscoff in a zipped-top bag and crush with a rolling pin until you have small crumbs with a few larger bits. Melt the butter with the brown sugar in a large nonstick skillet over medium-low heat. Add the cookie crumbs to the skillet and cook, stirring with a silicone spatula or wooden spoon, until lightly toasted, about 3 minutes. Remove from heat and allow to cool completely.
In a large bowl using a handheld or stand mixer fitted with a whisk attachment, whip the heavy cream, sugar, vanilla extract, and pumpkin pie spice on medium-high speed until medium peaks form, about 3 minutes. Medium peaks are between soft/loose peaks and stiff peaks, and are the perfect consistency for topping and piping on desserts. If you accidentally over-whip the cream, and it looks curdled and heavy, pour in a little bit more cold heavy cream, and fold it in gently by hand with a spatula until it smooths out. Spoon 1/4 cup (about 30g) of the crust into each jar. Spoon or pipe 1/2 cup (about 120g) of the pumpkin custard on top of the crust layer. I used Wilton 2A for the custard. Spoon or pipe whipped cream on top of each. I used Wilton 8B for the whipped cream. If desired, garnish each with a sprinkle of pumpkin pie spice or crushed cookie crumbs.
Serve immediately, or loosely cover the jars and refrigerate for up to 2 days.
