In a medium-large saucepan, bring the raspberries to a boil over moderate heat, continue boiling for about a minute or two, stirring and crushing the berries with a wooden spoon to release their juices. Using a fine mesh sieve, strain the raspberries over a clean medium-large saucepan, pressing down to get as much of the juice as you can. Place the raspberry juice back on the stove and bring to a simmer over moderate heat. Stir the juices occasionally at first, and then more frequently as the sauce begins to thicken to prevent scorching. Continue cooking until the juices are thickened to a jam-like consistency and have reduced to 1/2 cup. Cool in the refrigerator or in a water bath.
As the raspberries reduce, prepare the crust. Mix butter, cocoa, nibs, sugar, salt, vanilla in a medium bowl. Add flour and mix until blended. Scatter the mixture over the bottom of the tart pan, breaking it up with your fingers if the clumps hold together. Press the dough evenly with your fingers over the bottom and up the sides of a 10 or 11-inch tart pan with a removable bottom. Generously prick the bottom and sides of the crust with the tines of a fork. Bake in the lower third of an oven preheated to 350° F until the crust feels firm to the touch, about 20 to 25 minutes. Cool completely.
To prepare the filling, combine reduced raspberry juices, butter, cream, Chambord, vanilla extract, salt, and 3 cups of the powdered sugar in the bowl of a standing electric mixer. With the paddle attachment, stir on low speed to combine. Increase mixer speed and beat until well combined. The filling should look thick, shiny, and slightly sticky. If it feels too soft and gooey, add a bit more powdered sugar.
Scrape the filling into the cooled tart crust and spread it evenly to the edges, doing your best to smooth the top. Refrigerate while you prepare the ganache.
To make the ganache, place chopped chocolate in a measuring glass with a pour spout; set aside. Bring cream to a boil over medium heat. Pour hot cream over the chocolate; without stirring, let the cream-chocolate mixture sit for 2 minutes. After 2 minutes, stir slowly with a rubber spatula until the mixture is smooth and glossy.
Acting quickly, pour ganache over the raspberry filling. Carefully pick up the tart pan, tilting and rotating to spread the ganache to the tart’s edges as needed. If desired pop any large bubbles that form with a toothpick or the tip of a sharp knife.
Refrigerate uncovered for several hours to chill completely before serving. Serve chilled garnished with fresh raspberries, if desired.