Pre-heat oven to 350ºF (if baking crust). Add cookie crumbs, melted butter and a pinch of salt to a small bowl, then stir to combine. Tip into a 9-inch tart pan or pie plate, loosely spread into an even layer, then press firmly with the bottom of a drinking glass or measuring cup, making sure to press up the sides as well. Back until the crust smells toasty and is just barely golden brown, 12 to 15 minutes. Allow to cool completely before filling. If not baking, refrigerate the crust until ready to use (this can be done a day or two ahead, if wrapped in plastic).

Combine half the cream cheese (one 8 oz brick), 6 tablespoons sugar, 1 tablespoon lemon juice, vanilla, and a pinch of salt in the bowl of a stand mixer fitted with a paddle attachment. Mix at low speed until a thick paste forms, then turn up to medium until soft and smooth, scraping the sides as necessary. Then switch to the whisk attachment and pour in half the cream (1 cup). Mix at low speed to combine, then increase to high and whip until the mixture can hold stiff peaks, 3 to 5 minutes depending on the horsepower of your mixer. Scrape the filling into a bowl and set aside. Set the bowl back on the mixer (no need to clean it), and put the paddle attachment back on.

In the bowl of a food processor, grind the rest of the sugar (6 tablespoons) and the freeze-dried blackberries until powdery and fine, about 1 minute. To the mixing bowl, add the other 8 oz cream cheese, blackberry-sugar, 1 tablespoon lemon juice, cardamom, and the rest of the salt. With the paddle attachment, mix at low speed until a thick paste forms, then increase to medium speed and beat until soft and smooth. Scrape the bowl and beater with a flexible spatula, then switch back to the whisk attachment. Add the other cup of cream, mix on low speed to combine, then increase to high and whip until the mixture can hold stiff peaks, 3 to 5 minutes.

Working in heaped tablespoons, add dollops of the two fillings in an alternating pattern and swirl with the back of a spoon or smooth with an offset spatula. Cover with plastic wrap and refrigerate until the filling is fully chilled and firm, at least 6 hours, up to overnight. Top with fresh blackberries and a few mint sprigs and serve!