Preheat oven to 180 C / Gas 4. Grease and flour three 23cm round tins. Sieve together the flour, cornflour, bicarbonate of soda and salt. Set aside. In a small saucepan, heat water and 115g chocolate until melted. Remove from heat and allow to cool.
In a large bowl, cream 225g butter and 400g sugar until light and fluffy. Beat in 4 egg yolks one at a time. Blend in the melted chocolate mixture and vanilla. Beat in the flour mixture alternately with the buttermilk, mixing just until incorporated.
In a large glass or metal mixing bowl, beat egg whites until stiff peaks form. Fold 1/3 of the whites into the batter, then quickly fold in remaining whites until no streaks remain.
Divide batter into prepared tins. Bake in preheated oven for 30 minutes or until a skewer inserted into the centre of the cake comes out clean. Allow to cool for 10 minutes in the tins, then turn out onto wire rack.
Meanwhile, make the filling. In a saucepan, combine 200g sugar, evaporated milk, 115g butter and 3 egg yolks. Cook over low heat, stirring constantly until thickened. Remove from heat. Stir in coconut, pecans and vanilla. Cool until thick enough to spread.
Spread filling between layers and on top of cake. In a small saucepan, melt margarine and 30g chocolate. Stir until smooth and drizzle down the sides of the cake.