Preheat oven to 180 C / Gas 4. Line bottom of 20x30cm baking tin with baking parchment.
Microwave chocolate and water on high for 1 1/2 to 2 minutes. Stir halfway through. Stir until all is melted and smooth.
In a medium bowl, mix together flour, bicarbonate of soda and salt. Set aside.
In a large bowl, cream 225g butter and 400g sugar until light and fluffy. Add 4 egg yolks one at a time, beating well after each addition. Stir in chocolate and 1 teaspoon vanilla. Add flour mixture alternately with buttermilk. Beat after each addition until smooth.
In a separate bowl, beat egg whites on high until soft peaks form. Gently fold into batter. Pour into prepared tin.
Bake at 180 C / Gas 4 for 30 minutes or until skewer inserted into centre of cake comes out clean. Allow to cool.
Combine milk, 300g sugar, 175g butter, 4 egg yolks and 1 1/2 teaspoons vanilla in large saucepan. Cook and stir on medium heat for about 12 minutes or until thick and golden brown. Remove from heat. Stir in coconut and pecans. Cool to room temperature and spreading consistency. Spread over cake.