Place corn husks in a bowl, cover with boiling water, and soak for 30 to 60 minutes. Drain, place on a work surface, and cover with a clean, damp towel.

Place chicken in a pot, season with salt, and cover with water. Bring to a boil over medium-high heat, reduce heat, cover, and simmer until chicken is cooked through, 20 to 25 minutes. Drain chicken and shred with 2 forks.

Place tomatillo and serrano peppers in a pot while chicken is cooking, cover with water, and bring to a boil. Cook until tomatillos are soft and have changed color, but are not falling apart, 3 to 5 minutes.

Combine tomatillos, serrano peppers, onion, cilantro, and garlic in a blender; blend into a smooth sauce.

Heat 1 tablespoon butter in a saucepan over medium heat and add tomatillo sauce. Cook until sauce has slightly thickened, 3 to 5 minutes. Add shredded chicken and season filling with salt.

Beat remaining butter in a large bowl with an electric mixer until creamy. Add chicken broth, masa harina, baking powder, and 1 1/2 teaspoons salt and beat well until combined and the consistency of cookie dough. Test if the masa is ready by dropping a small ball of masa into a glass of cold water; if it floats, it's ready, if not, keep beating for a little longer.

Select 1 wide corn husk or 2 small ones. Spread about 2 tablespoons masa mixture onto the the corn husk, filling it up to 2 inches from the bottom and 1/4 inch from the top. Add 1 tablespoon of filling down the center of the masa mixture. Fold sides of husk together, one over the other. Fold the bottom of the husk over the seam of the 2 folded sides. Repeat with remaining husks.

Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add tamales with the open side up and cook until filling is heated through and separates from the husk, about 1 hour. Let tamales stand for 15 minutes before serving.