Pour the cold whipping cream into a chilled mixing bowl.
Add the instant pudding mix and, if desired, the icing sugar and vanilla. Start with 2 tablespoons of instant pudding mix. It helps stabilize the whipped cream so it holds its shape for piping, but too much can make it overly thick or give it a noticeable pudding flavour.
Beat on medium speed until the mixture thickens, then increase to medium-high and whip until stiff peaks form. Be careful not to overwhip.
Pipe onto a completely cooled pumpkin pie. Refrigerate until ready to serve.