In a large pot of lightly salted water, boil the potatoes until soft, about 20 minutes.
Drain potatoes and push them through a potato ricer into a large bowl.
Add butter, milk, egg, minced garlic, cheddar cheese, pancetta, and salt and pepper and mix well.
In a large nonstick skillet, heat 1 Tbsp oil over medium-high heat.
Form potato mixture into about 10 balls and gently flatten into pancakes.
Place in skillet in batches and cook until pancakes are golden brown on bottom, about 3 minutes.
Flip pancakes and cook until golden brown on other side, another 3 minutes.
Add more oil to the pan if necessary and repeat with remaining pancakes.
Top pancakes with Greek yogurt or sour cream and scallions.