In a large heatproof bowl, whisk the egg yolks and cornstarch together with a fork until well combined. Add a few drops of milk if the mixture is dry.

In a medium saucepan, combine the milk and sugar over medium heat and whisk until the sugar dissolves. Bring to a simmer (about 210°F). Remove from heat.

Slowly pour the warm milk mixture into the egg yolk mixture while whisking constantly to avoid scrambling the eggs. Add in the pumpkin and mix well.

Strain the mixture through a sieve back into the saucepan to remove any solidified egg bits. Return the saucepan to medium heat and whisk until the mixture thickens (about 5 minutes).

Remove from heat and whisk in the butter, vanilla, pumpkin pie spice, and a pinch of salt.

Cover the surface with plastic wrap and chill for at least 3 hours.

Crush the ginger snaps or Biscoff cookies in a zip-top bag. Melt the butter and brown sugar in a skillet over medium-low heat, add the crumbs, and cook until lightly toasted. Allow to cool.

In a large bowl, whip the cream, sugar, vanilla, and pumpkin pie spice until medium peaks form.

In each jar, layer 1/4 cup of cookie crust, followed by 1/2 cup of pumpkin custard, and top with whipped cream.

Garnish with extra pumpkin pie spice or cookie crumbs if desired.

Store in the refrigerator for up to 2 days, covered to keep fresh.