Cook and stir the curry powder and curry paste in a saucepan over medium-low heat until fragrant, about 2 minutes. Pour the coconut milk into the saucepan and mix well. Stir in the fish sauce, brown sugar, and 1 cup chicken stock.
Place the chicken thighs, peas, peppers, and carrots into the saucepan with the curry sauce. Bring the mixture to a boil over medium-high heat, then reduce the heat to low. Simmer until the chicken is cooked though, about 25 minutes.
Whisk together 1 tablespoon of cornstarch with 2 tablespoons of cold chicken stock. Stir cornstarch mixture into the curry. Mix the pineapple into the curry and cook until the sauce thickens, about 5 minutes.
