Preheat oven to 180°C. Grease and line the base and side of two 20cm round (base measurement) cake pans.
Stir the sugar, chocolate, butter, cocoa powder, Nutella, coffee, bicarbonate of soda and 2 cups (500ml) water in a large saucepan over medium heat for 3 mins or until sugar dissolves. Increase heat to high. Bring to the boil. Remove from heat. Pour into a large heatproof bowl. Set aside for 30 mins or until cooled to room temperature.
Add the flour and whisk to combine. Stir in the eggs. Divide between prepared pans. Bake for 1 hour or until a skewer inserted in the centres comes out clean. Cool in the pans for 5 mins, then turn onto a wire rack to cool completely.
To make the white chocolate buttercream, use an electric mixer to beat the butter in a bowl until very pale and creamy. Gradually add the icing sugar, beating well after each addition. Add milk and beat to combine. Add the white chocolate and beat until smooth.
Repeat to make the dark chocolate buttercream, adding Nutella with the dark chocolate.
Use a large serrated knife to level the top of each cake, then halve each cake horizontally. Place 1 cake layer on a serving plate. Spread with half the dark chocolate icing. Top with another cake layer.
Reserve half the white chocolate buttercream. Combine one-third of the remaining white buttercream with two-thirds of the dark chocolate buttercream. Spread evenly over the cake. Top with another cake layer.
Combine the remaining dark chocolate buttercream with the remaining white chocolate buttercream and spread evenly over the cake. Top with the remaining cake layer and reserved white chocolate buttercream. Decorate with chocolate eggs.