Preheat the oven to 350 degrees F. Lay 4 bacon slices on a rimmed baking sheet and bake until golden brown and crisp, about 10 minutes per side. Drain on paper towels; let cool completely.
Make the bacon salt (see below). Store extra in an airtight container in the refrigerator up to 2 weeks, or freeze up to 2 months.
Put the potatoes and the remaining 2 bacon slices in a large pot and cover with cold water. Bring to a boil, then reduce the heat to medium low and simmer until the potatoes are tender, about 25 minutes. Drain, discarding the bacon slices.
Combine the butter with the milk in the same pot and let melt over medium-low heat. Return the potatoes to the pot and mash them with a potato masher or fork. Add 1 tablespoon of the prepared bacon salt and stir to combine; transfer the potatoes to a bowl and top with more bacon salt.
How to make Bacon Salt:
Pulse the cooked bacon in a food processor until finely ground. Spread on paper towels and blot the excess grease.
Combine the bacon and salt in a small bowl and rub between your fingers until incorporated.