How to Make It
Place rice in a large bowl, and rinse under cold running water until water runs clear. Add water to cover rice, add shiitakes, and let stand at room temperature 8 hours or overnight.
Place corn husks in a large bowl. Add hot water to cover, and let stand until pliable, about 1 hour. Meanwhile, drain rice and shiitakes. Remove and discard shiitake stems; chop shiitake caps.
Bring 5 cups water just to a simmer in a medium saucepan. Remove from heat, and whisk in soy sauce, bouillon, clam juice, mirin, and salt.
Line a large rimmed baking sheet with parchment paper. In an extra-large skillet or wok, heat the grapeseed and sesame oils over high until nearly smoking. Add chorizo and cook, stirring occasionally, until fat is rendered and meat is just starting to brown, 3 to 5 minutes. Using a slotted spoon, transfer chorizo to a bowl. Pour off and discard all but 2 tablespoons drippings from skillet.
Add rice and shiitakes to skillet, and cook over medium-high, stirring constantly, until rice is opaque, about 3 minutes. Add 1/2 cup broth and cook, stirring constantly, until nearly absorbed. Continue adding broth, 1/2 cup at a time, stirring constantly, until broth is nearly absorbed between additions. The rice is done when translucent and very sticky, about 20 minutes.
Drain corn husks. Place 1 corn husk on a work surface with narrow end facing you. Brush wide half of husk with melted butter. Scoop about 1/4 cup rice mixture onto buttered half and spread in an even layer, leaving a 1/4-inch border on the sides and top. Spread 2 tablespoons chorizo down center of rice mixture. Fold the sides of the husk around filling, and then fold narrow end up to cover filling and close the tamale (the top should remain open). Repeat with the remaining husks, rice mixture, and chorizo to make a total of 16 tamales.
Arrange tamales, seam side down, in a large steamer basket (stack tamales in several layers if necessary). Fill a large saucepan with water to a depth of 2 inches. Place steamer basket in saucepan, and cover with a sheet of parchment paper and lid. Bring water to a boil, and reduce heat to medium-low. Steam tamales until rice is just firm, about 10 minutes. Uncover and let cool slightly before serving.