Pre-heat your oven to 220ºC/200ºC fan.
Using a sharp knife, cut slits in your potatoes. These should be about half-a-centimetre apart and go about two-thirds of the way through the potato.
Brush the tops of the potatoes with olive oil (about 1 tsp on each) and then sprinkle over a little salt.
Bake the potatoes for 1 hour (the cooking time may vary depending on the size of your potatoes). Every 20 minutes, brush the potatoes with the oil they're sitting in. If you notice the edges getting dark brown and crispy, loosely cover the potatoes in foil to stop them from burning.
While the potatoes are cooking thinly slice the Grana Padano Cheese (70g).
Once the potatoes have been baking for an hour, use a knife to check if they're soft in the centre, if not return them to the oven for a few more minutes. If they are, remove them from the oven and carefully push the sliced cheese into the slits. It doesn't need to go into every slit, about every 3 is fine.
Return the potatoes to the oven for 5 minutes to melt the cheese.
Remove the potatoes from the oven. Put a teaspoon of wholegrain mustard onto each potato and use the back of a spoon to spread it across the top. Roughly tear the Prosciutto di San Daniele and push it into some of the empty slits (as with the cheese, one in about every 3 slits is fine).