Step 1
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.
Step 2
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.
Step 3
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.
Step 4
Combine tomatillos serrano peppers onion cilantro and garlic in a blender; blend into a smooth sauce.
Step 5
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.
Step 6
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.
Step 7
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.
Step 8
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.