This easy custard has all the flavors you love for fall: pumpkin and warm spices, plus a little bit of tanginess and a thin topping of sweetened condensed milk. Pour it into pretty glasses for an easy, festive, make-ahead dessert.
Arrange 6 individual serving dishes, such as ramekins or dessert glasses or cups on a rimmed baking sheet. Make room for the baking sheet in your refrigerator.
Place the water in a shallow bowl and sprinkle the gelatin evenly over the surface. Let bloom for 5 minutes.
Meanwhile, heat the cream and brown sugar in a medium pot over medium heat, stirring occasionally to help the sugar dissolve completely, until the cream comes to a bare simmer. Measure out 1/3 cup of the sweetened condensed milk and reserve it; whisk the remainder into the pot, along with the pumpkin pie spice and salt.
Add the bloomed gelatin to the pot and stir until it is fully dissolved. Remove the pot from the heat and stir in the pumpkin puree, sour cream and vanilla. Transfer the mixture to a large heatproof container with a pour spout, such as a liquid measuring cup.
Divide the mixture among the prepared serving dishes. Refrigerate until fully set, at least 4 hours and up to 2 days (if storing longer than 8 hours, cover loosely with plastic wrap). Once the panna cottas have set, divide the remaining sweetened condensed milk over the portioned panna cottas to create a thin, even layer on each. Refrigerate until ready to serve.