Because the cranberry mixture benefits from time to set, this dessert is perfect for prepping in advance.
Prepare the cranberry topping: Measure 1/4 cup orange juice into a bowl. Sprinkle gelatin over juice and let mixture sit for 10 minutes.
Heat cranberries, remaining 1/4 cup orange juice, orange zest, sugar, and cinnamon in a medium saucepan. Bring to a simmer, stirring occasionally, over medium-high heat. Reduce heat to medium-low and cook, stirring often, until berries are tender and have just begun to burst, 3 to 4 minutes.
Pour cranberry mixture through a fine-mesh strainer set over a bowl. Add gelatin mixture to hot liquid in the bowl, and whisk to dissolve gelatin. Stir in strained cranberries, cover, and chill until thickened, at least 6 hours.
Prepare the chocolate-cookie crust: Preheat oven to 350°F. Combine ground cookies, butter, sugar, and salt in a bowl; stir until mixture has a wet-sand consistency. Press firmly and evenly into the bottom and up the sides of a 9-in. tart pan with a removable bottom. Place on a rimmed baking sheet and bake until crust is firm and appears dry, 10 minutes. Set aside to cool, about 15 minutes.
While crust cools, prepare the mascarpone filling: Combine mascarpone, confectioners' sugar, cocoa powder, and whipping cream in the bowl of a stand mixer fitted with a paddle attachment. Beat on low until just incorporated, 30 seconds. Spread into cooled crust, starting in center and spreading filling with an offset spatula to edges. Cover and chill until filling is firm, about 1 hour.
Spread cranberry topping over mascarpone filling; cover loosely and chill until ready to serve. Top each slice with a dollop of whipped cream, if using.