Make the cake layers
Preheat oven to 350°F. Lightly grease 3 (8-inch) round cake pans with cooking spray; line bottoms with parchment paper, and set aside. Sift together flour, cocoa, baking soda, and baking powder. Stir in salt.
Place brown sugar, butter, and mayonnaise in bowl of a heavy-duty stand mixer fitted with the paddle attachment; beat on medium speed until light and fluffy, about 3 minutes. Add eggs, 1 at a time, beating until fully incorporated after each addition. Stir in vanilla. With mixer running on low speed, add flour mixture alternately with buttermilk in 3 additions, beginning and ending with flour mixture. Beat until just incorporated after each addition. Divide batter evenly among prepared pans.
Bake in preheated oven until a wooden pick inserted in center comes out clean, 16 to 20 minutes. Let cakes cool in pans on wire racks 10 minutes. Remove cakes from pans, and let cool completely on racks, about 1 hour.
While cakes cool, make the buttermilk-chocolate frosting
Bring 1 inch of water to a simmer in a medium saucepan over medium. Combine unsweetened chocolate, bittersweet chocolate, and butter in a medium-size heatproof bowl. Place bowl over simmering water, ensuring base of bowl does not touch simmering water. Cook, stirring often, until mixture is melted and smooth, 8 to 10 minutes. Remove from heat, and let cool slightly, 10 to 15 minutes.
Combine powdered sugar, buttermilk, and vanilla in bowl of a heavy-duty stand mixer fitted with the whisk attachment; beat on low speed until smooth, about 1 minute. With mixer running on low speed, gradually add cooled chocolate mixture, beating until just incorporated, about 30 seconds. Set frosting aside, uncovered, and let stand, stirring occasionally, until completely cool, 1 to 2 hours. (Frosting should thicken as it cools. Do not chill.)
While buttermilk-chocolate frosting cools, make the pistachio praline
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.
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Make Ahead
Ricotta must be drained at least 24 and up to 72 hours ahead. Cake layers may be baked 1 day ahead, individually wrapped in plastic, and chilled. Pistachio praline may be made up to 1 week ahead and stored in an airtight container. Unglazed frosted cake may be assembled and chilled overnight.
