Bake meringues 1 hour. Turn off heat. Keeping oven door closed, leave meringues in oven overnight to dry (meringues will deflate slightly).
Chocolate chiffon cake:
Preheat oven to 325&Deg;F. Line bottom of 9-inch-diameter cake pan with 2-inch-high sides with parchment paper; spray parchment with nonstick spray. Combine 1/4 cup sugar and next 5 ingredients in medium bowl; whisk to blend.
Whisk 1/3 cup oil, egg yolks, and 2 tablespoons water in large bowl to blend. Stir in dry ingredients. Beat egg whites in another medium bowl until soft peaks form. Gradually add remaining 1/4 cup sugar, beating until stiff but not dry. Fold whites into yolk mixture. Transfer batter to prepared pan.
Bake cake until tester inserted into center comes out clean, about 25 minutes. Cool in pan 15 minutes. Turn cake out onto rack; peel off parchment. Cool completely. DO AHEAD: Can be made 1 day ahead. Wrap in foil; store at room temperature.
For mascarpone whipped cream:
Combine all ingredients in large bowl. Beat just until mixture holds soft peaks (do not overbeat or mixture will curdle). Cover and chill at least 1 hour and up to 2 hours.
Place chocolate in large microwave-safe bowl. Combine heavy whipping cream, 3 tablespoons water, unsweetened cocoa powder, and light corn syrup in heavy medium saucepan. Whisk over medium heat until blended and just beginning to boil. Pour mixture over chopped chocolate; let stand 1 minute, then stir until smooth. Whisk in butter. Let glaze stand until thick enough to spread, 15 to 20 minutes.
Place 1 meringue on platter, flat side down; spread 1/2 cup glaze over. Refrigerate until chocolate is firmly set, about 30 minutes. Spread half of mascarpone cream (about 1 1/2 cups) over chilled chocolate. Refrigerate 10 minutes. Place cake layer on work surface; spread with 1/2 cup marmalade, then remaining mascarpone cream. Carefully place cake layer atop meringue on platter. Top assembled cake with second meringue, flat side up. Spread 1/3 of glaze over top and sides of cake in thin even layer. Refrigerate until coating sets, about 30 minutes.
Heat remaining glaze in microwave just until pourable but not hot, 5 to 10 seconds. Carefully pour glaze over cake, spreading to coat evenly. Chill cake until glaze sets, at least 1 hour. DO AHEAD: Cake can be made 3 days ahead. Cover with cake dome and keep refrigerated.