For the Crust:

1. Pulse the cookies and salt in a food processor until fine crumbs form. With the machine running, stream in the melted butter. The mixture should be slightly moist and crumbly.
2. Press the mixture into a buttered tart pan (I used a 14-inch long rectangular one, but this would work in a 9-inch round pan too!), evenly pressing it into the corners and up the sides. Chill the crust in the fridge for at least 45 minutes before filling.

For the Filling:

1. Puree the berries in a food processor until smooth. Place a fine mesh strainer over a small saucepan and strain the mixture through to remove the seeds. Discard the seeds.
2. Place the pan over medium heat and add the powdered sugar, whisking well to combine. Bring to a boil and then reduce the heat and simmer for 10 minutes or until the berry juice is thickened and reduced by about half.
3. Meanwhile, place the chopped chocolate in a heat safe bowl. Pour the hot berry juice over the chocolate and allow to sit for about 30 seconds. Stir the mixture together until the chocolate is melted and smooth. Whisk in the vanilla and Chambord.
4. Stir in the mascarpone cheese until completely smooth. The filling should be thick and creamy but still pourable.
5. Pour the filling into the prepared crust. Decorate the top with fresh blackberries and then refrigerate for at least an hour.
6. Before serving, garnish with a few mint sprigs and a dusting of powdered sugar. Keep the tart chilled until ready to serve and store any leftovers in the fridge.