How to Make It
Stir together cornstarch and water in a small bowl. Stir together pumpkin, brown sugar, cane syrup, pumpkin pie spice, salt, and 1/2 cup of the heavy cream in a medium saucepan. Cook over medium-low, whisking constantly, until sugar dissolves, 3 to 4 minutes. Whisk in cornstarch mixture, and bring just to a boil. Reduce heat to low, and cook, whisking constantly, 2 minutes. Pour mixture through a fine-mesh strainer into a large bowl, and discard solids. Cool strained mixture completely, about 1 hour.
Beat 1 1/2 cups of the heavy cream and 2 teaspoons of the vanilla with an electric mixer fitted with whisk attachment on high speed until firm peaks form. Gently fold one-third of whipped cream into cooled pumpkin mixture until just blended; gently fold in remaining whipped cream until fully incorporated. Spoon about 2/3 cup into each of 6 serving bowls; cover with plastic wrap, and chill at least 4 hours or up to 1 day.
Beat remaining 1/2 cup heavy cream and 1/4 teaspoon vanilla in a medium bowl with electric mixer on high speed until foamy, about 3 minutes. Gradually add granulated sugar, and beat until firm peaks form. Dollop on each pumpkin mousse, and, if desired, sprinkle with pumpkin pie spice.