This extraordinary cake features airy layers of pistachio sponge cake a re nestled in cloudlike white-chocolate mousse layered with pistachio cream, and it's served with pistachio gelato.
Preheat oven to 350 degrees. Coat cake pans with cooking spray; cover bottoms with parchment, and coat again. Set aside. Whisk flour and baking powder in a bowl.
Put egg yolks and sugar into the bowl of an electric mixer fitted with the whisk attachment; beat on low speed until pale and thick, about 15 minutes. Transfer to another bowl; set aside.
Put egg whites into the clean bowl of an electric mixer fitted with the whisk attachment, and beat on medium-high until stiff peaks form. Fold into yolk mixture.
Fold in flour mixture, then the pistachios. Divide batter among prepared pans, filling each pan two-thirds full.
Bake until a cake tester inserted in centers comes out clean. Transfer to wire racks; let cool in pans slightly. Invert cakes onto racks. Remove parchment; reinvert. Cakes can be wrapped in plastic and stored up to 1 day.
To assemble: Cut each cake into 3 even layers, each about 3/4 inch thick. Brush each layer with syrup (see note below for amounts). Spread 2 layers with pistachio cream, then with mousse. Stack, placing plain cake layer on top. Frost tiers with buttercream.
Make this batter recipe 2 times for a 3-tiered cake to serve 70. Bake 1 cake for each tier, and cut into 3 layers.
Note: Batter amounts and cooking times are for 3-inch-deep round pans. 6-inch-diameter: 5 cups batter, 35 minutes; 8-inch-diameter: 8 cups batter, 45 minutes; 10-inch-diameter: 9 cups batter, 1 hour.
Filling and frosting amounts for each tier are as follows. 6-inch-diameter: 1/3 cup syrup per layer (1 cup total), 2 tablespoons pistachio cream per layer (1/4 cup total), 1/2 cup mousse per layer (1 cup total), 1 cup buttercream for exterior; 8-inch-diameter: 3/4 cup syrup per layer (2 1/4 cups total), 1/4 cup pistachio cream per layer (1/2 cup total), 3/4 cup mousse per layer (1 1/2 cups total), 1 1/2 cups buttercream for exterior; 10-inch-diameter: 1 cup syrup per layer (3 cups total), 6 tablespoons pistachio cream per layer (3/4 cup total), 1 cup mousse per layer (2 cups total), 2 cups buttercream for exterior.