Chocolate-and-Citrus Cassata

Place pistachios in a single layer centered on a parchment paper–lined rimmed baking sheet; set aside. Bring sugar and 1/4 cup water to a boil in a small saucepan over medium-high. Cook, stirring often, until sugar

 dissolves, about 2 minutes. Cook, without stirring, until sugar turns amber in color and registers 350°F on a candy thermometer, 8 to 10 minutes. Pour caramel over pistachios (do not stir). Let cool completely, about 20 minutes. Transfer hardened pistachio mixture to a cutting board; chop into pieces.

While praline cools, make the ricotta filling

Stir together ricotta, powdered sugar, orange zest, and vanilla bean seeds. (Mixture should be very thick and hold its shape when scooped.) Cover and chill until ready to use.

Assemble the cassata

Place 1 cake layer on a cake board. Spoon half of the ricotta filling (about 1 1/2 cups) over top of cake layer, spreading to leave a 1/2-inch border around edges. Top with a second cake layer; spread remaining ricotta filling to leave a 1/2-inch border around edges. Top with remaining cake

 layer. Stir buttermilk- chocolate frosting until smooth; spread a thin layer (about 2 1/2 cups) on top and sides of cake to seal in any crumbs and fill gaps between layers to encase filling. Chill cake, uncovered, until frosting is hardened, about 30 minutes. Spread top and sides of cake with a thick

 layer of remaining buttermilk-chocolate frosting, smoothing frosting with a bench scraper. Chill cake until frosting is firm and cold, at least 2 hours or up to overnight.

Make the dark chocolate ganache glaze

Combine bittersweet chocolate and butter in a 4-cup glass measuring cup with a spout; set aside. Bring cream and corn syrup to a low simmer in a small saucepan over medium-low. Pour cream mixture over chocolate mixture, and let stand 2 minutes. Stir glaze with a spatula until completely smooth; let stand at room temperature, stirring occasionally, until cooled to 100°F to 105°F, 4 to 10 minutes. (It should be pourable and fluid.)

Remove chilled cake from refrigerator, and place on a wire rack or inverted bowl inside a rimmed baking sheet. Pour glaze over top of cake, allowing glaze to flow down sides, making sure cake is completely covered. Let stand 1 minute, allowing excess glaze to drip off sides. Run an offset spatula around bottom edge of cake to remove any drips. Chill cake until glaze is set, about 1 hour. Carefully transfer cake to a cake stand or serving plate. Garnish with sliced clementines, other citrus, and desired amount of pistachio praline. (Reserve any remaining pistachio praline for another use.) Serve chilled.