Make crust: Preheat oven to 350 degrees. Process graham crackers, sugar, and salt in a food processor until finely ground. Transfer to a medium bowl, and stir in melted butter. Mix until combined.
Pour mixture into a 9-inch cake ring or springform pan set on a parchment-lined baking sheet. Press crumbs with your hand or the bottom of a glass to form an even layer on bottom and up sides of pan. Bake until crust just starts to turn golden brown, about 15 minutes. Let cool completely on sheet on a wire rack.
Make ganache: Melt 10 ounces bittersweet chocolate in a large heatproof bowl set over a pan of simmering water; keep warm. Meanwhile, bring cream just to a simmer in a medium saucepan over high heat; remove from heat. Whisk 1/4 cup hot cream into the beaten egg yolks, then whisk the yolk mixture into the remaining cream in saucepan. Whisk cream mixture into the melted chocolate, and set the ganache aside.
Coarsely chop remaining 5 ounces bittersweet chocolate, and transfer to a large bowl. Toss in 2 cups mini marshmallows and 1 cup whole toasted almonds. Scatter mixture all over bottom of cooled crust. Reserve 1 cup ganache for topping, and pour the rest evenly on top of the filling (do not overfill the crust). Refrigerate tart, uncovered, until firm, about 1 hour. Meanwhile, cover reserved ganache with plastic wrap, and set aside at room temperature.
Make topping: Toss together 3/4 cup mini marshmallows, 1/4 cup chopped toasted almonds, and 2 ounces chopped bittersweet chocolate; sprinkle over filling. Drizzle reserved 1 cup ganache over tart, and refrigerate 10 minutes more before cutting into slices.
To toast almonds, spread in a single layer on a rimmed baking sheet; bake at 350 degrees until fragrant, 10 minutes.