Preheat the oven to 175°C (350°F). Pulse the graham crackers into a fine crumb in a food processor. Add the sugar and softened butter and pulse a dozen more times until damp and sandy. Press into the bottom and up the sides of a 9" pie plate. Bake for 10 minutes, press down again and cool completely, leaving the oven on. Whisk the lime zest and egg yolks until airy. Add the lime juice and condensed milk and whisk until smooth. Pour into the cooled crust and bake 12 minutes until set, not brown. Cool to room temperature, then chill at least 2 hours. Whip the cold cream to fluffy peaks, then whip in the powdered sugar, vanilla and a pinch of salt. Pile onto the cold pie, leaving a border, and zest a lime over the top. Chill another couple of hours, then slice and garnish with thin lime slices.