Note: We recommend asking your butcher to bone the whole chicken for you, keeping the breast intact (alternatively, cut chicken into quarters, leaving bones inside). Ground wattleseed is an Australian spice that is not common in the United States; Jewels calls it her "secret ingredient." For extra heat, add another teaspoon of ancho chili powder.
To make ancho-porcini rub: Mix ancho chili powder, ground wattleseed, porcini mushroom powder, garlic powder, and salt in a small bowl.
Wash chicken; dry with paper towels, removing as much moisture as possible. Drizzle 3 tablespoons olive oil all over chicken, and rub in well to lightly coat entire surface.
Cover chicken with ancho-porcini rub, massaging it in to infuse the flavors. Set aside 10 to 15 minutes.
Preheat oven to 350°. Heat a large ovenproof skillet over medium-high heat. Add remaining olive oil. Sear chicken skin side down until deep brown and crisp, about 6 minutes. Flip and repeat. Finish cooking in oven, about 30 more minutes depending on size of chicken. It is done when a thermometer inserted into the thickest part of the thigh registers 165°. (Alternatively, grill chicken: Preheat grill to medium-high. Cook chicken skin side down 8 to 10 minutes, then flip and cook for same amount of time on other side. If using pieces with bones, cook about 20 minutes on each side.)
Let chicken rest 10 minutes before serving. Arrange on plates with mashed yams on the side; serve.