Bake the vanilla cake according to package instructions in a standard square pan.
Cool the cake completely on a wire rack then cut into uniform half-inch cubes.
In a large bowl, combine the pumpkin puree and pumpkin pie spice.
Gently fold the whipped cream into the pumpkin mixture until uniform in color.
Transfer the mousse into a piping bag for easier assembly.
Place two cake cubes into the bottom of each glass jar.
Pipe a layer of pumpkin mousse over the cake cubes.
Drizzle one teaspoon of caramel sauce over the mousse layer.
Repeat the layers until the jars are filled, finishing with a dollop of mousse and caramel drizzle.
Refrigerate for at least one hour before serving to allow textures to set.