Chop the garlic finely. Peel and chop the apple. Chop the onion. Cut the chicken into cubes and season with salt and pepper. Heat 2 tablespoons of butter over medium heat. Add chicken and fry, stirring occasionally, until golden brown. Remove from heat and place chicken on a plate. Set aside.
Add butter to the same skillet and reduce heat to medium. Add chopped onion and garlic and stir. Cook until light brown. Sprinkle flour over onions. Stir, then add curry powder. Cook over medium heat for 1 minute, stirring constantly.
Pour in the chicken broth. Stir, cook for five minutes. Add milk and cream, 2 teaspoons of salt, pepper and sugar, then cook for another 5 minutes.
Add chicken and chopped apples and cook for another 5 to 10 minutes. Turn off heat and let stand 5 or 10 minutes before serving. Add cayenne pepper if desired.