Let chicken sit at room temperature for 30 minutes before preparing. Positiion a rack in the middle of your oven. Preheat oven to 425 degrees.
Rinse the chicken inside and out with cold water. Use paper towels to thoroughly dry the chicken. This is super important to get crispy skin.
Add the butter along with the garlic powder, thyme, and 1 teaspoon sumac to a medium. Mix until combined. (Make sure to use a medium bowl as you will use it again later.)
Place the chicken in a large cast iron skillet. (Mine is 12 inches but anything up to 15 inches will work. You don't want the surface to be too large or the juice could burn.) Again make sure the chicken is very dry.
Sprinkle the chicken all over with salt and pepper. Use your hands or a spatula to spread the softened butter all over the chicken. Sprinkle with a little bit more salt.
Place the skillet in the oven and set the timer for 35 minutes.
While the chicken cooks, prep the veggies. Half the carrots lengthwise and then cut into 2-3-inch pieces. Cut the fingerling potatoes in half lengthwise. Add the veggies to the same bowl you mixed the butter in. Add the olive oil, 1/4 teaspoon salt, and remaining 1/2 teaspoon sumac. Toss to combine.
When the timer is up, pull the chicken out. Spread the veggies around the chicken. Place back in the oven for another 35 minutes. After 35 more minutes, start to check the temperature of the chicken. It's done when the thickest part of the thigh reaches 165 degrees on an instant read thermomter. Pull the chicken out and let rest at least 15 minutes before carving.
To carve: I gently cut the skin where the leg connects to the body and then gently pull off the thigh and drumstick. Find the middle of the breast bone. Use a large sharp butcher's knife to cut down the breast bone on one side. Take the knife and slice the bottom of the breast to remove the whole breast in one piece. Repeat on the other breast. Cut each breast into slices.
Serve the chicken with the vegetables and spoon juice on top.