Preheat the oven to 425 degrees.
Place each puff pastry sheet onto a parchment lined baking sheet. Cover the puff pastry with another sheet of parchment then cover the top with another baking sheet. Bake in the oven for 20-25 minutes or until golden brown. Let cool.
Cut the puff pastry down to fit into a 9x13 pan. Line the pan with parchment paper making sure there is overhang on each end. Place one sheet of the puff pastry at the bottom and reserve the top for later.
In a large saucepan, whisk the sugar, corn starch, pumpkin spice, and salt together until the corn starch has no clumps. Add in the cream and milk and place over medium heat, stirring constantly until the mixture just barely starts to boil. Turn off the heat.
Whisk the egg yolks and pumpkin into a heat resistant bowl. Slowly pour the hot cream mixture into the egg yolks while whisking. Once the cream mixture is about half gone, pour the cream and egg yolk mixture back into the cream mixture and combine. Turn the heat back onto medium/high and whisk constantly until the mixture starts to boil. Continue to mix while boiling for 1 minute then remove from the heat. The mixture will start to look weird and might separate a bit. That’s okay, we’re getting the custard to set to the right consistency.
Remove the custard from the heat and whisk in the butter until melted.
Pour the custard onto the prepared pastry lined pan. Smooth out until even then place the final puff pastry sheet on top. Cover and let set in the fridge for at least 4 hours.
Remove from the pan by lifting both ends of the parchment paper. Cut into 12 squares and dust with powdered sugar.