Pat the wings dry and season with salt, garlic powder and pepper, then arrange them in a 6-quart slow cooker.
Cover and cook on high for 2 1/2 to 3 hours until the thickest wings reach at least 165°F.
Move the wings to a foil-lined rimmed pan and broil 5 to 8 minutes, turning once, until browned and crisp at the edges.
Melt the butter in a small pan, cook the minced garlic for 1 minute and remove from the heat.
Toss the hot wings with garlic butter, Parmesan and parsley if using, then serve immediately.