Season the chicken liberally with salt and pepper on both sides. Heat the butter and oil in a large pot or Dutch oven over medium-high heat. In batches, saute the chicken in the fat, until nicely browned, about 3 minutes on each side. Turn with tongs or a spatula. When a batch is done, transfer it to a plate and continue to saute all the chicken in batches. Remove the last chicken to the plate and add all of the garlic to the pot. Lower the heat and saute for 5 to 10 minutes, turning often, until evenly browned. Add 2 tablespoons of the Cognac and the wine, return to a boil, and scrape the brown bits from the bottom of the pan. Return the chicken to the pot with the juices and sprinkle with the thyme leaves. Cover and simmer over the lowest heat for about 30 minutes, until all the chicken is done.