Preheat the oven to 375°F.
Slice potatoes into 1/4 inch thick rounds and layer flat in a large skillet. (I like to use a mandoline to get the slices as even as possible.)
In a saucepan, on medium-low heat, melt butter and add the flour to make a roux. Whisk constantly for 5 minutes.
As the roux gets darker, slowly add buttermilk, whisking as you pour.
Add ranch seasoning, black pepper, and salt. Remove from heat.
Stir in 1 cup shredded cheese (I like to switch to a spatula instead of whisk), making sure it is melted.
Pour the cheese sauce over the potatoes that are in the skillet.
Cover the skillet and cook for about an hour.
Uncover the potatoes, add the second cup of cheese, and bake for 25-30 more minutes, or until the cheese is browned.