For the crust, preheat the oven to 350 degrees F. Combine all the ingredients until the crumbs are evenly moistened and press them into the bottom and up the sides of a 9-inch tart pan or pie plate. Bake the crust 10-12 minutes until lightly browned and fragrant (don't overbake or the crust will become crumbly). Cool completely on a wire rack while the filling is prepared.
For the filling, mix the sugar and cornstarch together in a medium saucepan. Gradually whisk in the cream, lime juice, and lime peel. Stir in the butter pieces. Bring the mixture to a boil over medium heat, whisking constantly. Reduce the heat to a simmer and whisk constantly until the mixture is thick and bubbly, about the consistency of pudding, 5-7 minutes. Remove the pan from the heat and cool the mixture to room temperature, stirring occasionally to avoid a skin forming over the top. If you need to speed things up, fill a bowl with ice and water and place the bottom of the pot in the ice water (taking care not to let water spill into the pot) and stir until cooled. Fold the sour cream into the cooled lime mixture. Spread evenly in the cooled crust.
For the topping, whip the cream, sugar and vanilla together until soft peaks form. Gently fold in the 1/4 cup sour cream. Spread over filling. Chill the tart/pie for at least 4 hours before serving and up to 24 hours. Serve with fresh lime slices, twisted, if desired.