Preheat the oven to 180 °C. Grease and flour a round mold.
Melt the chocolate in the boiling water and let it cool down. Sift the flour with the bicarbonate and salt apart. Mix in another container the milk with the vinegar so that it is cut.
Beat the butter in a bowl until it gets creamy, add the sugar little by little, and keep beating until it gets spongy. Without stopping beating, add one by one the yolks, the melted chocolate, and the vanilla. Turn off the mixer and add the sifted flour, alternating with the milk, and beating with a spatula or wooden shovel.
Beat the egg whites apart until they form firm peaks. Wrap them in the dough.
Pour the dough into the greased pan and bake in the preheated oven until it passes the stick test, approximately 35 minutes. Take the cake out of the oven and leave it in the mold for 10 minutes, then remove the mold and place it on a cooling rack to cool completely.
Heat the milk along with the sugar, yolks, butter, and vanilla in a pot over medium heat. Cook, stirring frequently until thickened. Remove from heat and add the coconut and nuts. Let it cool down.
Cut the cake in half, horizontally, to have two thin layers.
Cover the surface of the lower layer with the icing and arrange the upper layer on top. Cover everything with the rest of the icing.