Place chicken breasts in a gallon-sized resealable plastic bag, along with ¼ cup buffalo sauce and ½ packet ranch seasoning. Close bag and toss until chicken is thoroughly coated.
Grease a large skillet with cooking spray. Add chicken and cook over medium heat, 6 to 8 minutes per side.
In a medium-sized saucepan, melt butter, then add ¼ cup buffalo sauce and cream cheese, whisking to combine. Slowly add in milk, whisking constantly. Add remaining ranch seasoning and keep whisking until a sauce has formed. (It will thicken as it cools.)
In a large pot of boiling salted water, cook pasta according to package instructions. Drain, then toss in sauce. Dice chicken into bite-sized pieces and toss into pasta mixture.
Garnish with green onions before serving.
