Heat 2 tablespoons oil in a large skillet over medium heat and cook and stir onion until translucent, about 5 minutes. Stir in garlic and ginger; cook and stir just until fragrant, about 1 minute. Stir cumin, 1 teaspoon salt, cayenne pepper, cinnamon, and turmeric into the onion mixture; fry until fragrant, about 2 minutes.
Stir tomato sauce into the onion and spice mixture, bring to a boil, and reduce heat to low. Simmer sauce for 10 minutes, then mix in cream, paprika, and 1 tablespoon sugar. Bring sauce back to a simmer and cook, stirring often, until sauce is thickened, 10 to 15 minutes. Transfer sauce to a crockpot.
Heat vegetable oil in a separate skillet over medium heat. Stir chicken into the hot oil, sprinkle with curry powder, and sear chicken until lightly browned but still pink inside, about 3 minutes; stir often. Transfer chicken and any pan juices into the sauce. Cook on high for 2 hours or on low for 4 hours.