In a small saucepan whisk together milk, water and cornstarch. Add butter, bouillon, salt, pepper, onion powder and garlic powder.
Bring to a low boil, stirring frequently. If using uncooked chicken, season it with salt and pepper and add to the pot, Reduce heat and cook until chicken is cooked through and sauce has thickened to gravy consistency. If using leftover cooked chicken, stir it in at the end.
To serve, add a spoonful of rice to a bowl. Pour chicken gravy on top. Add desired toppings.
Make ahead instructions: Make the rice, cook the chicken, and make the sauce up to 1 day ahead of time. Store rice and gravy separately in the refrigerator. Freezer instructions: The Hawaiian haystacks sauce will freeze well. Allow it to cool completely once it thickens and store it in a freezer safe container for up to 2 months. Thaw in the refrigerator overnight and reheat on the stovetop.