Preheat the oven to 350 F and prepare our molds. Mix the milk chocolate in a double boiler and let it get cold. I used El Rey 41% cocoa. If you don't have buttermilk, heat the milk a little, the lemon or vinegar, and set aside. When you heat the milk, it will cut faster.
In a bowl, sift the flour and mix with cocoa, baking powder, baking soda, and salt.
In a second bowl, mix the coffee and buttermilk or milk.
In the mixer, blend the butter until smooth and creamy. Add the sugar until the mixture is a pale yellow, 3 to 5 minutes. Clean the sides of the bowl with the spatula and add the eggs one at a time. Add the vanilla and finally the melted chocolate. Whisk until the chocolate is well integrated with the mixture.
Now add the buttermilk/coffee mixtures and the flour in three batches, starting and ending with flour. Flour/buttermilk/flour/buttermilk/flour. Once everything is integrated, we are going to place the mixture in the molds. I do make sure they are the same to weigh the molds with the combinations to make sure they all consider the same.
Bake for 30 to 35 minutes and let stand for 10 minutes before unmolding.
Frosting
Preheat the oven to 350 F and toast the walnuts for 8 minutes, let cool and chop into small pieces.
In a small saucepan, combine sugar, evaporated milk, egg yolks, butter, and salt. Cook over medium heat, constantly stirring until it begins to boil and thicken. This will happen after about 5 minutes. It will take a little more golden color. Remove from the heat and add the coconut and walnuts and stir to integrate well.
To assemble the cake, divide the frosting into three equal parts. Place the first cake with the top side down, cover it with frosting, repeat with the second layer, and finally, the third. On top, you can decorate with whole nuts, shredded or flaked coconut, melted chocolate.