Season the chicken breasts on both sides with salt and black pepper.
Heat olive oil in a skillet over medium heat. Sear the chicken breasts for 2 to 3 minutes per side until lightly browned. Transfer the chicken into the slow cooker liner.
Turn the skillet heat down to medium-low. Add the butter and chicken base, stirring constantly until the butter is completely melted and combined.
In a small bowl, mix together the flour, onion powder, garlic powder, and a pinch of extra salt and pepper. Sprinkle this mixture into the skillet. Whisk continuously for about 30 seconds to cook out the raw flour taste, letting it bubble gently.
Slowly pour the chicken stock into the skillet a little at a time, whisking vigorously to prevent lumps. Scrape up any browned bits stuck to the bottom of the pan. Bring to a simmer until it starts to thicken into a smooth gravy.
Pour the gravy over the chicken breasts in the slow cooker. Cover and cook on High for 3 to 4 hours (or on Low for 6 to 7 hours) until the chicken is easily shreddable.
Using two forks, shred the chicken directly inside the slow cooker pot. Stir the shredded meat into the gravy to coat it thoroughly. If using heavy cream, stir it in now and let it warm through for 5 minutes.
Spoon hot mashed potatoes onto plates, ladle a generous amount of the shredded chicken and savory gravy over the top, and serve immediately alongside sweet dinner rolls.