Bring 2 cups chicken stock to a boil with the salt, pepper and butter. Mix polenta and parmesan with the milk and gradually whisk the mixture into the boiling chicken stock. Whisk constantly until it returns to a boil, then lower heat and cook about 30 minutes, whisking frequently to prevent lumps.
For the Chicken
Heat a large skillet over medium-high heat. Add oil; swirl to coat. Sprinkle chicken with salt and pepper. Add chicken to pan, skin side down; cook 5 minutes or until browned. Turn chicken over; cook 2 minutes. Add garlic to pan; cook 1 minute. Add 2 cups stock, raisins, vinegar, and olives to pan; bring to a boil. Reduce heat, and simmer, partially covered, 12 minutes or until chicken is done. Increase heat to medium-high; cook, uncovered, 3 minutes or until liquid is slightly thickened. Divide polenta evenly among 4 plates; top with chicken thighs and sauce. Sprinkle with parsley.