Make cake: Preheat oven to 450 degrees, with racks in upper and lower thirds. Coat two 10 1/2-by-15 1/2-inch jelly-roll baking pans with cooking spray. Line with parchment paper; coat with cooking spray. Dust with flour, tapping out excess. In a bowl, whisk together flour and teaspoon salt.
In a large bowl, using an electric mixer, beat eggs, yolks, and sugar on medium-high, 2 minutes; increase speed to high and beat until pale and thick, 5 minutes. Sift flour mixture over egg mixture; with a large rubber spatula, fold together until almost blended. Pour butter down side of bowl, folding to combine. Divide batter between pans; smooth with a table knife. Immediately transfer pans to oven and bake until cakes are golden brown and springy when pressed, 6 to 7 minutes, rotating pans halfway through. Let cool completely.
Make filling: In a large bowl, stir together egg yolks, mascarpone, sugar, cocoa, orange zest (if using), and pinch of salt. In another large bowl, using an electric mixer, beat egg whites and pinch of salt until soft peaks form. With a large rubber spatula, fold one-third whites into mascarpone mixture; fold in remainder.
Invert cakes onto a work surface and gently peel off parchment. Using a serrated knife, trim cakes to measure 10 by 15 inches, then cut each crosswise into thirds. Place 1 cake layer, golden side up, on a platter; brush with brandy and spread with 1 cup filling. Repeat with remaining cake, brandy, and filling, ending with a layer of brandy-brushed cake. Refrigerate until set, about 30 minutes.
Make Chocolate Glaze. Pour over top of cake and spread so it drips down sides (or spread along sides). Cover and refrigerate 1 hour (or up to 2 days).