Add the potatoes to a pot and cover with water. Season with salt and bring to a boil. Once at a boil, cook until a fork can easily pierce through the potatoes, 10 to 15 minutes. Drain and add to a bowl.
Place the bacon in a cast-iron pan over medium heat. Allow the bacon to render and crisp, about 5 minutes. Reserve the fat.
Put the buttermilk and heavy cream into a saucepan and scald. Reserve until needed.
Smash the potatoes just a little bit, and then season with salt and pepper. Add the butter, a touch of the heated buttermilk/heavy cream, the scallions and 1 tablespoon bacon fat and smash. Taste and re-season if needed, adding more buttermilk/heavy cream if desired. Fold in the rendered bacon, reserving some for garnish. Garnish with the reserved bacon and scallions.