Chicken Chasseur with Herbs and Cognac

Ingredients

• 3 russet potatoes
• 4 tablespoons unsalted butter, divided
• 4 tablespoons olive oil
• 3 pounds bone-in chicken pieces
• 2 teaspoons kosher salt
• 1/2 teaspoon ground black pepper
• 1 pound cremini mushrooms, sliced
• 1/4 cup shallots, finely diced
• 1 1/2 tablespoons garlic, freshly minced
• 1/2 cup cognac or brandy
• 1/2 cup dry white wine
• 3 tablespoons double concentrated tomato paste
• 2 Roma tomatoes, diced
• 1 1/2 cups chicken stock
• 1/2 cup beef stock
• 1/4 cup sour cream
• 2 tablespoons fresh tarragon, chopped
• 2 tablespoons fresh parsley, chopped