Heat oven to 190C/fan 170C/gas 5. Boil the potatoes whole, in their skins, in gently simmering water until tender – about 30-40 mins depending on size. Meanwhile, fry the pancetta cubes until golden.
Drain the potatoes and set aside. Put the milk and most of the butter in the potato pan to warm. Peel the potatoes, cut into big chunks, then tip back into the pan with the milk. Mash them (as long as you’ve used a floury potato you will get a light and fluffy mash by using an electric hand whisk, or a masher). Mix in the pancetta, eggs, garlic and all of the other ingredients except the breadcrumbs and thyme.
Use the remaining butter to butter the bottom and sides of a 23cm springform tin. Coat the inside of the tin with about ¾ of the breadcrumbs and fill with the potato mixture. Smooth down the surface and sprinkle with the remaining breadcrumbs, pressing them gently into the potato.
Bake for about 1hr 10 mins until the potato cake is set, with a slight wobble in the middle. Let it rest for 5 mins then loosen from the sides with a knife before releasing the tin. Slide onto a plate and sprinkle with the thyme leaves. Serve hot or warm. Can be prepared up to the end of step 3 the night before and kept covered in the fridge.