Preheat oven to slow, 150 C. Grease and line base and sides of three 20 cm springform round cake pans. In a large saucepan, combine sugar, water, butter, chocolate, coffee and vanilla. Stir on low until smooth. Transfer to a large bowl. Cool 30 mins. Lightly whisk in eggs. Stir in combined sifted dry ingredients.
Divide mixture evenly between pans. Bake 1 hour until cooked when tested with a skewer. Cool in pan. Remove cakes from pan, level tops. Set one aside. Grease and line two of the cake pans. Return remaining two cakes to pans.
MOUSSE In a heatproof bowl over a saucepan of gently simmering water, melt chocolate with ½ cup cream, stirring until smooth. Whisk in gelatin to dissolve. Cool slightly, then stir in yolks. In a bowl, whip remaining cream until soft peaks form. Gently fold cream into chocolate mixture.
In a clean, dry bowl, beat egg whites into chocolate mixture until combined. Chill 1 hour until beginning to thicken. Pour evenly over cakes in pans. Chill overnight to set. When ready to serve, in a small saucepan, combine cream and dark chocolate, stirring on low until smooth. Set aside to cool slightly.
Remove cakes from pans. Place one mousse-topped cake on a plate. Dollop with a little ganache. Repeat with remaining mousse cake and ganache, finishing with reserved cake. Pour rest of ganache over, using an offset palette knife to drop ganache down the sides. Chill 30 mins until set.
Meanwhile, line an oven tray with baking paper. Dip a drinking straw into melted chocolate and place on tray. One at a time, dip edges of some Easter eggs and a few halved Ferrero Rochers into chocolate then attach to straw, finishing at the top bendy part. Chill 15 mins until set.
Insert bamboo skewers into straw, then push into centre of cake at an angle. Arrange rest of eggs and Ferreros on top of cake. Brush top of straw with melted chocolate and attach the party bag for a truly “gravity-defying” effect!