Preheat the oven to 200°C/400°F and place the potatoes onto a sheet pan. Place in the oven and allow to bake for an hour or until a knife can easily be inserted.
When the potatoes are cooked, remove from the oven and allow to cool for 10 minutes. Carefully halve the potatoes. Scoop out the cooked flesh and discard the skins.
Mash the potatoes with a potato masher or ricer.
Heat the milk, garlic, salt, pepper and butter together until the milk starts to simmer. Remove from the heat and pour into the potatoes. Mix well.
Stir in the Parmesan cheese then season to taste and serve.