Pre heat oven to 160°C (fan forced) and grease and line 2 x 20cm cake tins with baking paper.
In the bowl of an electric mixer, beat together the butter and sugar until smooth and creamy (approx. 5 minutes).
Add vanilla, eggs, flour, baking powder, cocoa and water. Combine on low speed and then increase speed to medium and mix for 3 minutes until mixture is smooth and paler in colour.
Evenly divide the cake batter into the two pans and bake for 60 minutes.
Allow cakes to cool in the tin for 10 minutes before turning out onto a wire rack to cool completely.
To make Nutella buttercream in 3 shades of ombre brown, first make a big batch of vanilla buttercream: In a large bowl using a stand mixer fitted with a paddle attachment, beat the butter on medium speed until smooth, about 2 minutes. Stop the mixer and add icing sugar, cream or milk and vanilla extract. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2 minutes. Add up to ½ cup more icing sugar if buttercream is too thin or another tablespoon of cream if it is too thick.
Evenly divide the buttercream into 4 bowls. Leave one portion white for the very top of the cake. Into one bowl, add ½ cup Nutella to make the darkest shade for the bottom layer. In the next bowl add 1/3 cup Nutella to make a lighter shade for the middle layer and to the last bowl add ¼ cup Nutella to make the lightest shade of buttercream for the second last layer.
On a cake board, begin assembling the cake layers with the darkest shade of Nutella buttercream on the base layer. Repeat with each of the cake layers spreading the darkest to lightest as you build up, finishing with the white buttercream on the very top of the cake.
Decorate the top with easter eggs, speckled eggs and small easter bunnies.