Mixed Mushroom Tamales

INGREDIENTS

• 1/2 oz dried porcini mushrooms (1/2 cup)
• 1/2 teaspoon baking powder
• 2 cups fine-ground masa harina (dry corn masa; 9 1/4 oz)
• 36 large dried corn husks (3 oz), separated and any damaged husks discarded
• 1 lb mixed fresh mushrooms such as shiitake caps, white button, and cremini, coarsely chopped (4 1/2 cups)
• 1 teaspoon dried epazote (optional), crumbled
• 1 tablespoon minced garlic
• 2 sticks (1 cup) unsalted butter, softened
• 1 teaspoon sugar
• 2 1/2 cups very hot water
• 1/2 small onion, finely chopped (1/4 cup)
• 2 teaspoons salt
• 1/2 teaspoon black pepper