Preheat the oven to 350F.
Set a 10-inch, cash iron pan over medium high heat and add the chorizo. Cook the chorizo, stirring often, until most of the fat has rendered and the chorizo is well caramelized, about 4-5 minutes. Add the butter, leeks and potatoes, stirring, until leeks are soft and potatoes get a little brown on them, 4 to 6 minutes.
Meanwhile in a large bowl whisk together the eggs, milk or cream, a pinch of salt, a pinch of pepper and 1/2 cup grano padano until eggs are frothy. Set aside.
Pour the egg mixture over the chorizo mixture and continue to cook stirring gently and lifting with a rubber spatula to let the eggs flow underneath. Do this until the edges are set but the middle still is loose, about 3 minutes.
Remove from the heat and sprinkle the remaining grano padano, another pinch of pepper and salt over the top.
Bake the frittata for about 25 minutes or until lightly browned and puffed and the cheese is golden brown.
Remove from the oven, top with fresh basil, cut and serve.