Prepare the Potatoes by washing, peeling if desired, and cutting into chunks.
Boil the potatoes in salted water until fork-tender, about 15-20 minutes.
Drain the potatoes and return to pot; dry out over low heat for 1 minute.
Add butter, garlic powder, salt, and black pepper; mash until mostly smooth.
Gradually mix in warmed milk or cream until creamy; keep warm.
Pat chicken dry and season with salt and pepper; pound to even thickness.
Heat olive oil in skillet over medium-high heat and sear chicken 3-4 minutes per side until cooked through; set aside.
Add butter to same skillet; melt and sauté garlic for 1 minute until fragrant.
Pour chicken broth into skillet, scraping browned bits; stir in heavy cream, thyme, and rosemary.
Simmer sauce 5-7 minutes until slightly thickened.
Return chicken to skillet; spoon sauce over and simmer 2-3 minutes to warm through and coat.
Serve chicken over mashed potatoes, spooning extra sauce on top.
Optional: Garnish with fresh parsley or chives and serve immediately.