1. Prepare Cake: Preheat oven to 350°. Line bottom of a 9-inch springform pan with parchment paper; coat pan with cooking spray.
2. Beat 3/4 cup butter at medium speed with a heavy-duty electric mixer until creamy (about 2 minutes). Gradually add 1/2 cup granulated sugar, beating until light and fluffy. Add egg yolks, 1 at a time, beating just until yellow disappears after each addition. Add vanilla and melted chocolate, beating on low speed just until blended. Gradually add flour, beating just until blended. In a separate bowl, beat egg whites at high speed with an electirc mixer until foamy; gradually add remaining 1/4 cup granulated sugar, beating until soft peaks form. Fold in one-fourth of egg whites into batter; gently fold in remaining egg white mixture. Pour batter into pan.
3. Bake at 350° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool completely in pan on a wire rack (about 2 hours). Run a knife around edge of cake to loosen; invert onto work surface. Cut horizontally into 3 layers.
4. Prepare Apricot-Orange Filling: Process apricot preserves in a food processor 30 seconds or until smooth. Transfer to a small saucepan, and cook over medium-low heat until melted, stirring often. Remove from heat, and stir in 1/4 cup liqueur.
5. Brush 1/2 cup apricot mixture over 1 cake layer; top with second cake layer. Brush 1/2 cup apricot mixture; top with remaining cake layer. Brush top with remaining 1/4 cup apricot mixture. Set cake on a wire rack over a rimmed baking sheet.
6. Prepare Glaze: Place chopped chocolate in a medium bowl. Combine whipping cream and corn syrup in a small saucepan; bring to a simmer over medium heat. Pour over chocolate; let stand 1 minute. Whisk until smooth; stir in butter. Pour glaze over cake, covering top and sides completely. Chill cake 1 hour or until glaze is set.
7. Just before serving, prepare Grand Marnier Cream: Beat 1 cup whipping cream with an electric mixer until foamy; add powdered sugar and orange liqueur, beating until soft peaks form. Cut cake into 12 wedges. Top with Grand Marnier Cream. Garnish, if desired.