If you plan to make the frozen whipped cream, do this before preparing the cocktail. The whipped cream can be made up to two days in advance and stored in the freezer until you’re ready to use it. In a bowl, combine heavy cream and confectioners' sugar, then whip until stiff peaks form. Transfer the whipped cream to a pastry bag fitted with a large tip (any shape), and pipe it into round towers on a parchment-lined tray (you'll make four). Place the tray in the freezer immediately and freeze for at least 30 minutes.
1/2 cup heavy cream*, 2 tbsps confectioners' sugar
Dip the rims of your martini glasses in water or crème de cacao, then shake off any excess liquid. Next, dip the rims into cinnamon sugar to coat.
2-3 tsps cinnamon-sugar, for the rim
Chill the prepared martini glasses in the freezer or fridge for at least thirty minutes before serving.
For the cocktail, fill a shaker halfway with ice. Add the vodka, crème de cacao, heavy cream, and pumpkin pie spice. Shake well for at least fifty vigorous shakes. Strain the mixture into the chilled martini glasses. Top each drink with a frozen whipped cream tower and lightly sprinkle with additional pumpkin pie spice.
1/4 cup vanilla vodka such as Absolut brand, 1/4 cup Creme de Cacao liqueur (clear)**, 1/2 cup heavy cream*, 1-1/2 tsps pumpkin pie spice, more for sprinkling on top