Begin by preparing the potatoes, which form the foundation of this dish. Cut 3 lbs of russet potatoes into 2-inch chunks and place them in a large pot with 2 teaspoons of kosher salt and enough cold water to cover them completely.
Bring the water to a boil over high heat, then reduce the heat and simmer until the potatoes are very tender, approximately 20 to 25 minutes.
Once cooked through, drain the potatoes thoroughly and return them to the pot, which will help remove excess moisture for a fluffier final texture.
While the potatoes cook, prepare the bacon and dairy mixture. Cook 3 ounces of bacon (about 6 slices) in a large skillet over medium heat until crisp, which takes 5 to 8 minutes.
Transfer the cooked bacon to a paper towel-lined plate to cool, then crumble or chop it into bite-sized pieces. In a separate step, combine 3/4 cup whole milk, 8 tablespoons of unsalted butter cut into pieces, and 1/2 teaspoon salt.
Warm this mixture in the microwave for 45 seconds until the butter is melted and the milk is warm.
Pour the warm milk and butter mixture over the drained potatoes and add 1/4 teaspoon black pepper. Mash the potatoes to your desired consistency—whether you prefer them smooth or with some texture remaining.
Finally, fold in the crumbled bacon gently to distribute it evenly throughout, then top the finished mashed potatoes with 4 ounces of crumbled blue cheese. Serve immediately while the cheese begins to melt into the warm potatoes.