Chocolate inhibits yeast growth, so this dough takes a bit longer to rise but the tender rich rolls with the decadent icing make it worth the wait!
Stir the yeast into warm water in a large mixing bowl and let stand 5 minutes. Heat milk and butter until warm (100° to 110°F) and add to bowl. Add 1 cup flour, sugar, cocoa, salt, egg, egg yolk and vanilla. Beat for 2 minutes with electric mixer until very smooth. Add enough of the remaining flour to make a soft dough. Turn out onto a lightly floured surface and knead about 8 to 10 minutes, until the dough is smooth and elastic. Place dough in a large greased bowl, cover and let rise about 1-1/2 to 2 hours, or until dough has doubled in size.
Punch dough down. On a lightly floured surface, roll dough into a 15 x 10-inch rectangle. Spread with softened butter. Combine brown sugar and cinnamon and sprinkle over butter. Roll up from long sides like a jelly roll. Cut into 12 equal pieces. Arrange rolls, cut sides up in a greased 13 x 9-inch pan.
Cover pan loosely with greased plastic wrap and let rise until doubled, approximately 1 hour.
While rolls are rising, prepare frosting. Combine evaporated milk, sugar, egg yolks and butter in a large saucepan. Cook over medium low heat, stirring constantly until mixture is thick and comes to a boil. Remove from heat and stir in vanilla. Add nuts and coconut. Refrigerate until ready to frost rolls.
Bake in preheated 350ºF oven for 25 to 30 minutes or until lightly browned. Remove from oven and cool on wire racks for 20 minutes.
Spread prepared frosting over rolls and serve immediately.