Rub chicken with oil and season with salt and pepper. Add chicken breasts to a large pot and cook over medium-high heat for 15-20 minutes until cooked through (165 degrees F), flipping once. Remove to a cutting board and slice thinly. Toss with barbecue sauce and set aside, keeping warm while you make the macaroni.
In a large pot, combine macaroni, milk and salt. Over medium-high heat (you don't want it too high, because milk will scald and leave a bitter flavour), bring the milk to a simmer, stirring frequently. Reduce to medium heat and cook, stirring frequently, for 10-15 minutes until noodles are cooked and milk is thickened (if sauce is thick before pasta is cooked, slowly add in a little bit more milk). Stir in cheese.
Spread macaroni and cheese into serving dish and top with BBQ chicken. Garnish with extra cheese and green onions if desired.
Notes
You could also cook the chicken in the oven, in a frying pan, or on the grill. For me, I wanted easy and moist, so I went the slow cooker route. *You want to stir frequently and watch closely, because there isn't extra moisture in the pot and you don't want the noodles stuck to the bottom. You also don't want your milk to scald on the bottom of the pot.