In a mixing bowl, whip the heavy cream until stiff peaks form. You can use an electric mixer or do this by hand with a whisk. Make sure the cream is very cold for best results. Set in the refrigerator to set for 15 minutes.
In a small bowl, dissolve the instant coffee or espresso powder in 2 tablespoons of hot water. Stir until it forms a smooth coffee concentrate. Let it cool for a few minutes.
In another bowl, combine the sweetened condensed milk, pumpkin pie spice, and vanilla extract. Mix until all the ingredients are well combined. Add the coffee concentrate to the milk mixture and stir until fully incorporated. Gently fold the whipped cream into the coffee mixture. Do this in batches, folding carefully to maintain the mousse's light and airy texture.
Spoon the pumpkin spice latte mousse into serving glasses or bowls. Cover them with plastic wrap and refrigerate for at least 2 hours, or until the mousse has set. Garnish and Serve: Before serving, you can top the mousse with a dollop of extra whipped cream and a sprinkle of cinnamon if desired.