1. Lightly butter a 10-inch pie plate and set aside.
2. Make the cranberry purée: In a medium saucepan combine cranberries, sugar, and water. Bring to a simmer over medium heat, stirring frequently to dissolve the sugar. Reduce heat and let the berries bubble gently for about 12–15 minutes, stirring occasionally, until the mixture thickens and most berries have burst.
3. Purée and chill: Transfer the hot cranberry mixture to a blender or food processor (or use an immersion blender) and process until smooth. Cool the purée completely — speed it up by placing it briefly in the freezer, but don’t let it freeze solid.
4. Prepare the crust: While the cranberries cook, mix gingersnap crumbs with the melted butter until the crumbs are evenly moistened. Press the mixture firmly and evenly into the bottom and up the sides of the prepared pie plate. Chill in the refrigerator while you finish the filling.
5. Make the mousse: In a food processor or a mixer, combine the cooled cranberry purée with the cream cheese and vanilla. Process or beat until the mixture is silky and free of lumps. (If using a stand mixer, soften the cream cheese first.)
6. Fold in whipped topping: Gently fold the thawed Cool Whip (or whipped cream) into the cranberry–cream cheese mixture until homogenous and no streaks remain. Work carefully to keep the mousse light and airy.
7. Assemble and chill: Spoon the mousse into the chilled gingersnap crust, smoothing the top with an offset spatula. Refrigerate for several hours or overnight to firm up and make clean slices.
8. Finish and serve: Top with piped or spooned whipped topping, scatter sugared cranberries and mint leaves if desired, then slice and enjoy.

Pro tip: Let the cranberry purée cool fully before combining with cream cheese — hot purée can make the cream cheese grainy.