Make the topping:

Sprinkle 1½ teaspoons unflavored gelatin over ¼ cup cold water in a medium bowl and let it soften for 10 minutes. Put 1 12-ounce bag fresh or frozen cranberries in a saucepan with ¾ cup sugar and ¼ cup water. Bring to a boil, stirring to dissolve sugar. Lower heat to a simmer and cook an additional 5 minutes or so — you want to soften the cranberries but keep their plump shape.

Drain the cranberries over the bowl in a mesh colander. Stir the juice with the gelatin to combine, then add the cranberries and stir gently. Cover and refrigerate until the mixture thickens, about 6 hours.

Make the tart crust:

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Heat the oven to 350 (175 C) degrees.
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Pulse 30 chocolate wafer cookies, ¼ cup sugar, and ¼ teaspoon salt in a food processor until fine crumbs form. Add 6 tablespoons melted butter and pulse until the mixture is moistened and holds together when you pinch it.

Press the crumbs onto the bottom and sides of a 9-inch tart pan with a removable bottom. Bake the crust 10-12 minutes. Cool completely on a rack.

Make the filling:

Blend 1 pound mascarpone cheese, 1 cup powdered sugar, and ⅓ cup unsweetened cocoa powder together until smooth, by hand or using the paddle attachment of a heavy duty mixer. Spread the filling over the cooled crust and refrigerate at least 2 hour.

To serve, spoon the cranberry mixture over the chilled filling.