Preheat oven to 180 C / Gas 4. Grease and flour three 20cm round tins. Sieve together the flour, cornflour, baking powder, bicarbonate of soda and salt. Set aside. In the top of a double boiler, heat chocolate, stirring occasionally, until chocolate is melted and smooth. Remove from heat and allow to cool to lukewarm.
In a large bowl, cream together the butter and 300g sugar until light and fluffy. Beat in the eggs one at a time, then stir in melted chocolate and 1 teaspoon vanilla. Beat in the flour mixture alternately with the buttermilk, mixing just until incorporated. Pour batter into prepared tins.
Bake in the preheated oven for 30 to 35 minutes or until a skewer inserted into the centre of the cake comes out clean. Let cool in tins for 10 minutes, then turn out onto a wire rack and cool completely. Let cool.
Meanwhile make the icing. In a large saucepan, combine evaporated milk, 200g sugar, egg yolks, 115g butter and 1 teaspoon vanilla. 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. Use to fill and top the cake.