Crust :

1. Preheat the oven to 350°F / 180°C with the fan oven.
2. Start by finely mixing the graham crackers in a food processor until you have a fine crumbs.
3. Melt the soft butter in a saucepan over medium heat or in the microwave.
4. Mix together the melted butter and cracker crumbs in a medium bowl with a wooden spoon until the crumbs have soaked into the butter.
5. Pour this crumbs mixture into a 9-inch diameter pie plate and press down with your hands or with the back of a tablespoon to form the pie crust.
6. Bake for about 7 - 10 minutes, then let cool completely to room temperature.

Lemon filling :

1. Preheat the oven to 320°F / 160°C fan oven.
2. Place the sweetened condensed milk with the sour cream, lime juice and zest in a large bowl and mix with a whisk until you have a smooth cream.
3. Pour this mixture into the pie plate containing the prepared graham cracker crumbs and smooth the top with an angled spatula.
4. Bake for about 15-20 minutes until the cream firms up.
5. Let the pie cool at room temperature for about 20 minutes before placing the pie in the refrigerator for at least 3 hours.

Whipped cream :

1. Place the cold heavy cream with the powdered sugar in a medium bowl and beat with an electric mixer at medium speed for about 5 minutes. Until you obtain a firm whipped cream.
2. Fill a pastry bag fitted with a star tip with whipped cream and decorate the top of the tart.
3. Add extra lime zest and enjoy.