Grease and line the base and sides of two 22cm (base) springform pans. Combine butter, sugar, water, cocoa and bicarbonate of soda in a medium saucepan. Stir over a medium heat until butter is melted. Increase heat to high, bring to boil. Remove as soon as mix comes to the boil. Transfer to a large bowl. Cool for 30 minutes, whisking occasionally, mixture should be luke-warm.
Preheat oven to 160°C fan forced. Whisk the eggs into the chocolate mixture. Sift the flour over the mixture and whisk gently until smooth. Divide between the pans. Bake for 40-45 minutes, or until a skewer inserted into the centre comes out clean. Stand in cake pans for 20 minutes. Release side and remove cakes to a wire rack to cool completely.
Using a serrated knife, cut each cake in half horizontally. Whip the cream to stiff peaks, fold in the mascarpone, vanilla and icing sugar. Set 8 strawberries aside, hull and slice the rest. Place one cake layer onto a cake plate, spread with one-third of the cream and one-third the sliced strawberries. Sandwich together with a second cake layer. Repeat layers two times with cream, strawberries. Top with final cake layer.
For the chocolate icing; Pour the cream in a microwave-safe bowl. Heat 1-2 minutes on High/100% until hot. Do not boil. Add chocolate and stir with a metal spoon until smooth.
Spoon over the cake, allowing it to drizzle a little. Top with reserved strawberries. Allow to set 15 minutes before slicing.