Preheat the oven to 180°C (160°C fan-forced). Lightly grease three 20cm round cake tins and line the bases with baking paper.
Using an electric stand mixer, combine eggs, buttermilk, warm water, oil and vanilla, slowly beating until well combined. Add flour, sugar, cocoa, bicarbonate soda and salt, mixing slowly until mixture is smooth and well combined.
Using a set of scales, evenly divide batter among the three prepared cake tins.
Bake cakes in preheated oven for 35 minutes or until a skewer comes out of the centre of each cake clean. Remove from oven and allow to cool for 10 minutes, before turning onto a wire rack to cool completely. Using a bread knife remove the domed tops from each cake, creating three even layers.
For the buttercream mousse, place chocolate, cream, syrup, zest and liqueur together in a medium saucepan over a very low heat, stirring constantly until chocolate has melted, making sure mixture does not become too hot. Remove from heat and allow to cool to room temperature.
Once chocolate mixture has cooled, place in mixing bowl of an electric stand mixer with a balloon whisk attachment. Beat mixture on a medium-high speed, adding butter a little at a time until mixture becomes pale, smooth, glossy and mousse like.
To ice cake, place each cake layer on a baking tray, making sure the top layer is cut side down. Place butter cream in a disposable piping bag, snipping the end. Pipe little icing kisses by pushing down as you squeeze the icing and then lift up to create a point. Repeat this process until all the cake layers are covered in icing kisses. Place baking trays in refrigerator for 30 minutes, to allow buttercream mousse to set.
To serve, place the bottom layer of the cake in the centre of a cake stand or serving plate. Place a few berries around the outside. Carefully place the middle layer on top and again place some berries around the outside. Finally place the top layer carefully on top, making sure it is centred. Carefully scatter berries and edible flowers over the top. The cake is best refrigerated until serving, making it easier to slice, as the mousse remains set.
*NOTE: Edible flowers are available from good fruit and vegetable suppliers and may need to be specially ordered.