Preheat oven to 375 F/190 C.
Line a baking sheet with foil then spray with non-stick spray.
Mix the breadcrumbs and grated cheese in a small bowl; set aside. Lightly beat the egg in another small bowl; set aside.
Mix the mashed potatoes, onion (if using), chives and bacon. Mixture should be soft but hold together. If it seems too dry, add 1 tbs of the sour cream then check the texture. If it's still too dry, add the remaining sour cream.
Form a 2-inch/5 cm ball around a cheddar cheese cube.
Roll each ball in the beaten egg then dredge in the breadcrumb mixture. Place on prepared baking sheet.
Bake for 15-20 minutes or until cheese starts to melt out.
AIR FRYER--follow the above but reduce baking time to 8-10 minutes (time may vary with the power of your air fryer.