Rinse the chicken and pat dry. Heat the butter in another small pan and toast the peppercorns for one minute, set aside. In a large saucepan, stir together yogurt, 2 teaspoons cornstarch and next six ingredients. Add the chicken and mix well. Marinate 30 minutes at room temperature. Cover and bring to boil over medium heat. Reduce the heat and simmer, turning the chicken occasionally. Chicken should simmer about 35 minutes. If there is a lot of liquid, leave the lid off while cooking. Reduce the heat, shaking pan occasionally until there is just one cup of liquid and chicken is no longer pin inside. If the sauce does not seem thick enough, mix the last teaspoon of cornstarch with teaspoon cold water and add it to the remaining liquid in pan and boil, stirring constantly. Scatter the cilantro over the top and lower heat. Pour the peppercorns over the chicken and gently mix in. Cover and let stand 5 minutes before serving. Serve the thighs over rice with the sauce.