Heat butter and oil in a large skillet over medium heat. Add onion and peppers, and cook until onions are soft and translucent. Stir in cooked chicken and garlic, and sprinkle with sugar. Cook, stirring for about 3 minutes, then stir in maple syrup, and cook for about 5 more minutes, until caramelized.
Stir in curry paste and heavy cream, and reduce heat to low. Simmer uncovered for 10 to 15 minutes, or until the sauce reaches your desired thickness.