3 tablespoons slivered blanched almonds
6 tablespoons sugar
1 1/4 cups (spooned and leveled) unbleached all-purpose flour
1/4 teaspoon kosher salt
6 tablespoons cold unsalted butter, cut into pieces
12 ounces bittersweet chocolate, coarsely chopped
1 1/4 cups heavy cream
1 teaspoon pure vanilla extract
Lightly sweetened whipped cream, for serving (optional)
Preheat oven to 350 degrees. Make dough: In a food processor, pulse almonds until finely ground. Add sugar, flour, and salt; pulse until combined. Add butter, pulsing until coarse crumbs form with no large butter lumps (dough should clump together when pinched with fingers).
Immediately transfer dough to a 9-inch tart pan with a removable bottom. Using a measuring cup, evenly press dough into the bottom and up the sides of pan.
Bake in center of oven until golden brown and firm to the touch, about 20 minutes. Transfer to a wire rack to cool completely, about 1 hour.
Make ganache: Place chocolate in a large mixing bowl. In a small saucepan, bring cream to a boil. Pour hot cream, through a sieve, over chocolate. Stir until smooth and creamy in texture. Mix in vanilla.
Pour chocolate mixture into center of cooled tart shell (if chocolate is lumpy, pass through a sieve). Let stand until set, about 2 hours, or refrigerate for 1 hour. Serve with whipped cream, if desired.