**Cook the Potatoes:**
Bring a large pot of salted water to a boil. Add the cubed potatoes and cook until tender, about 15-20 minutes. Drain well.
**Mash the Potatoes:**
Return the drained potatoes to the pot. Mash until smooth using a potato masher or ricer.
**Prepare the Mixture:**
In a saucepan over medium heat, melt the butter and sauté the minced garlic until fragrant, about 1 minute. Add the milk or heavy cream and warm gently. Pour this mixture into the mashed potatoes and stir until fully combined.
**Add Cheese and Seasoning:**
Mix in the Parmesan cheese, egg, salt, and black pepper. Stir until the mixture is smooth and well blended.
**Preheat the Oven:**
Preheat your oven to 400°F (200°C) and lightly grease a muffin tin with non-stick cooking spray.
**Fill the Muffin Tin:**
Scoop the mashed potato mixture into the muffin cups, filling each one to the top. Use the back of a spoon to smooth the tops.
**Bake:**
Bake for 20-25 minutes, or until the tops are golden brown and slightly crisp.
**Serve:**
Allow the potato cups to cool for 5 minutes before carefully removing them from the muffin tin. Garnish with chopped parsley if desired and serve warm.