Heat the oven to 350°F (175°C). Toast the pecans on a baking sheet for 7 to 9 minutes, cool and chop. Grease three 9-inch round pans, line with parchment, grease the parchment and dust with flour.

Pour the boiling water over the chopped sweet baking chocolate, wait 1 minute and stir until smooth. Cool until just warm.

Whisk the flour, baking soda and salt. Beat the butter and 1 1/2 cups sugar on medium-high for 3 to 4 minutes, until fluffy. Beat in the egg yolks one at a time, then the vanilla and melted chocolate.

On low speed, add the flour mixture in three parts alternating with the buttermilk in two, starting and ending with flour. Mix just until combined.

In a clean bowl, whip the egg whites until foamy, gradually add the remaining 1/4 cup sugar and whip to soft, glossy peaks. Stir one-third into the batter, then gently fold in the rest.

Divide the batter between the pans (about 2 1/3 cups each) and bake 24 to 30 minutes, until the tops spring back and a toothpick comes out with a few moist crumbs. Cool 10 minutes in the pans, turn out and cool completely.

For the frosting, whisk the evaporated milk, egg yolks, brown sugar and salt in a saucepan and add the butter. Cook over medium heat, stirring constantly, 10 to 14 minutes, until bubbling, thickened and at least 160°F (71°C).

Off the heat, stir in the vanilla, coconut and toasted pecans. Cool about 1 hour, stirring now and then, until thick and spreadable.

Level the layers if needed. Stack them with one-third of the frosting between each layer and on top, leaving the sides bare. Let set 30 minutes before slicing.