In a small bowl stir together the curry powder, cumin, turmeric, 2 teaspoons of salt, and pepper. Add the chicken to a large bowl and season all over with the seasoning mixture.
Pour the buttermilk over the top and cover and refrigerate the chicken in the brine for at least 4 hours and up to overnight.
Heat oven to 425ºF (220ºC). Remove the chicken from the brine and rinse then pat completely dry. Let sit at room temperature for 30 minutes then add chicken to a roasting pan.
Use fingers to slide the butter under the skin. Roast the chicken for 20 minutes then remove from the oven.
Lower the oven to 400ºF (200ºC). Toss the vegetables in the olive oil and arrange on pan around the chicken.
Season both the vegetables and chicken with salt and continue to roast until the skin is crispy and the chicken is completely cooked through and tender, about an additional 30 to 40 minutes (the internal temperature should read 165ºF).