You will start by making the caramel, but you'll want to have all the ingredients for the caramel and also the pumpkin filling out and measured beforehand, so you can work quickly once the caramel comes together.I like to put the butter, cream, and vanilla right next to the caramel pot, so it's easy to access when the sugar melts. Heat granulated sugar in a medium saucepan over medium heat, stirring constantly with a high heat resistant rubber spatula or spoon.
Sugar will form clumps and eventually melt into a thick brown, amber-colored liquid as you continue to stir. Be careful not to burn.Once sugar is completely melted, immediately remove from the heat. Next add the butter. Be careful in this step because the caramel may bubble rapidly when the butter is added.Stir the butter into the caramel until it is completely melted. Very slowly, drizzle in heavy cream while stirring. Since the heavy cream is colder than the caramel, the mixture may rapidly bubble and/or splatter when added.Stir for one minute until the mixture thickens. Add vanilla and mix well.
Next, add in the brown sugar, salt, and pumpkin and mix well.
Now you'll need to temper in the eggs, so they don't cook in the mixture.Crack both eggs into a small bowl and whisk well. Then, using a small spoon, scoop several spoonfuls of the caramel mixture into the egg mixture and mix well.
Continue and add one more spoonful at a time, until the egg mixture is warm and has come to temperature of the caramel pumpkin filling.
Once the egg mixture is warm, pour back into the larger saucepan and mix well.
Once fully mixed, pour the pumpkin caramel mixture over the shortbread baked in the pan.
Be sure the oven is set down to 325°F, then carefully place the pan back in the oven.
Bake for 20-25 minutes until the edges are slightly puffed and the inside only slightly jiggles/wobbles. (You don't want it to slosh around, you want a slight jiggle.)
Once edges are slightly puffed and middle only slightly jiggles, remove from oven.
Let the bars rest at room temperature for 20-30 minutes, then place in the fridge to chill until cold (about 3-4 hours, up to overnight).
Once chilled, slice bars with a very sharp knife (wiping the knife between each cut, to create clean lines).