Add all the ingredients to a small saucepan over medium heat. Whisk until the cornstarch is dissolved and bring to a simmer until thickened. Transfer to a bowl and set aside.
Make the chicken:
Add the shallot, garlic, coriander, turmeric, garam masala, ginger, brown sugar and salt to a blender. Add 4 tablespoons water and blend until you get a chunky paste. Add the paste to the chicken and coat well. Marinate for 2 hours, or overnight, refrigerated. Add oil to a Dutch oven or high-sided heavy pan. Fill no more than 1/3 full and heat to 350°F/175°C.
Add the chicken to a high sided pan along with the marinade and toss to coat well. Bring to a simmer and cook at a simmer for 5 minutes per side until the water has evaporated and there is just paste left. Turn off heat and leave in the pan. When the oil is ready, add the chicken in small batches (2 or 3 at a time), to avoiding crowding the pan. Fry for 10 minutes until golden brown, turning halfway through. The internal temperature of the chicken using a meat thermometer should be no lower than 165°F/75°C. Remove and drain on a rack or paper towel. Serve warm with the sauce.