Preheat oven to 375°F (190°C).
Make the garlic sauce: remove any loose skins and crush 1 head of garlic with 1/2 teaspoon salt in a mortar and pestle until a paste forms.
Mix in 1 tablespoon olive oil, 1/4 cup cold water, and 1 tablespoon chopped fresh parsley into the crushed garlic until combined; set aside.
Combine the dry rub: in a small bowl mix 1 teaspoon garlic powder, 1 teaspoon onion powder, 1 teaspoon salt, 1 teaspoon black pepper, 1 tablespoon paprika, and 1 tablespoon dried basil.
Rub the dry mixture all over the whole chicken, including under the skin where possible, and place the chicken on the roasting rack in the roaster. If your pan has no rack, pour a little oil into the bottom of the pan first.
Cover the chicken loosely with aluminum foil and roast for 2 hours total, removing the foil after 1 hour. Start checking doneness after 1½ hours; the chicken is done when an instant-read thermometer in the thickest part of the thigh reads 165°F (74°C).
Thirty minutes after the chicken goes into the oven, prepare the potatoes: in a bowl combine 1/2 of the reserved garlic sauce with 2 tablespoons melted butter and 2 tablespoons olive oil.
Toss the 8 cleaned and cubed potatoes in the garlic-butter-oil mixture and season with salt and pepper to taste.
Arrange the potatoes around the chicken on the roasting rack or in a separate single-layer pan if needed. Cover with foil again and continue baking, removing foil after 1 hour to allow browning.
When the chicken reaches 165°F (74°C) and the potatoes are tender and browned, remove from the oven and let the chicken rest for 10 minutes before carving.
Serve the carved chicken with the roasted potatoes and use any leftover garlic sauce as a drizzle or dip.