Special equipment: kitchen twine
Preheat the oven to 475 degrees F. Place the butter in a bowl and mash with a fork until smooth. Add the matcha and 1 teaspoon salt and stir until evenly combined.
Slide your forefinger gently under the skin of the chicken and loosen it from the breasts and thighs. Using a spoon, stuff scoops of matcha butter underneath the skin of the breasts and thighs and press it so it spreads out evenly. Take any remaining matcha butter from the bowl (or that doesn't get underneath the skin), and rub it all over the outside of the chicken. Sprinkle the chicken generously with more salt and pepper.
Peel off the outer layers of each leek until you have at least 8 inches of white and light green parts to each. Roughly chop enough tough green parts to make a handful, then trim off and discard the rest. Cut the white and light green leeks crosswise into 1-inch-thick rounds and rinse thoroughly to remove any dirt on them. Place the leek rounds and olive oil in a bowl, sprinkle with salt and pepper and toss gently to coat. Arrange the rounds on a rimmed baking sheet flat-side down so they form a "bed" for the chicken. Stuff the chopped greens inside the chicken along with the garlic. Tie the legs together with kitchen twine and place the chicken on the bed of leeks.
Bake the chicken for 20 minutes, then reduce the oven temperature to 400 degrees F and continue baking until the chicken is golden brown, the skin is crisp and an instant-read thermometer inserted into the thigh reads 160 degrees F, about 1 hour.
Remove the baking sheet from the oven and let the chicken rest for 10 minutes. Transfer the chicken to a cutting board and cut into serving pieces. Scrape the leeks and pan juices into a bowl and toss together until evenly coated. Transfer the leeks to a serving platter and arrange the chicken on top of the vegetables. Spoon any pan juices over the chicken and sprinkle with the parsley.
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