Cake: In bowl, cream butter with sugar until fluffy. Beat in eggs, one at a time, then vanilla. Beat in sour cream. Combine flour, baking soda and chocolate; add in 3 additions to creamed mixture alternately with 2 additions of bananas, mixing just until flour is incorporated. Spoon into 2 greased 8-1/2-inch (2.25 L) springform pans, smoothing tops. Bake in 350°F (180°C) oven for 35 to 40 minutes or until tops spring back when touched. Let cool on rack for 20 minutes. Remove sides of pans; let cool completely.
White Chocolate Icing: In saucepan, bring half of the cream to boil; pour over chocolate in bowl, whisking until melted. Add vanilla. Refrigerate until chilled, about 1 hour, whisking often. On medium speed, beat chocolate mixture just until ridges hold shape. Beat remaining cream just until soft peaks form; gently fold into chocolate mixture. With serrated knife, slice cakes in half horizontally. Place a top layer, cut side up, on plate.
Assembly: Spread top with 3/4 cup (175 mL) icing; cover with single layer of bananas, leaving 1/2-inch (1 cm) border. Cover with 1 bottom cake layer, cut side down; repeat with icing and bananas. Repeat with remaining top layer, cut side up, and some of the icing and bananas. Top with remaining cake layer, cut side down. Using palette knife, cover cake smoothly with remaining icing. Refrigerate until firm, about 1-1/2 hours.
Dark Chocolate Ganache: Meanwhile, bring cream to boil; pour over chocolate in small bowl, whisking until melted. Let cool for 20 minutes or until room temperature and still pourable. Pour over centre of cake, spreading to edge with clean palette knife, if necessary, and letting some flow down sides. If ganache becomes too firm, gently re-warm over hot, not boiling, water until pourable. Refrigerate until firm, about 40 minutes, or for up to 1 day. To serve, drizzle melted chocolate with fork over remaining bananas; let set. Arrange on cake. Slice with serrated knife.