1) Stir the apple cider vinegar into the milk and set aside.
2) Preheat the oven to 425°F. With a fork, deeply pierce the russets a few times all around, then place directly on the middle oven rack. Bake for 45 minutes to 1 hour, until a paring knife easily passes through the center.
3) About halfway through the russets’ cooking time, add the Yukon Golds to a large pot, along with the salt, garlic, and bay leaves. Add enough water to cover the potatoes by about an inch. Turn the heat to high. Once boiling, reduce the heat to a simmer, and cook the potatoes for 10 to 15 additional minutes, until a paring knife easily passes through the centers. It’s best to check several of the potatoes for doneness, but you don’t have to jab every single one.
4) At this point, your russets and Yukons have finished cooking at the same time. Remove the russets from the oven and immediately cut them in half crosswise to allow excess steam to escape.
5) Drain the Yukons and discard the water, garlic cloves, and bay leaves. Return the Yukons to the pot and place over low heat for 1 minute to help evaporate excess water, then turn off the heat.
6) Place a ricer over a large mixing bowl. Place each russet half into the ricer with the cut side down and the skin side facing you. As you press the potato, the flesh pushes through the holes and drops into the bowl, while the skin is left in the ricer. Have a fork or tongs standing by to easily extract the skin after each potato is processed. Pass all of the Yukon Golds through the ricer into the same bowl.
7) Once all potatoes are processed, you’ll have a fluffy, white and gold pillow of potato-y goodness, Stir the melted butter into the potatoes until incorporated.
8) Add the white pepper and about two- thirds of the buttermilk mixture. Fold it into the potatoes and give them a taste.
9) Add a little more buttermilk, 1 tablespoon at a time, remembering to stir minimally so as to not release the gummy starch.
10) Once you’ve reached the texture you like (you will probably have 2 to 4 tablespoons of buttermilk left over), season with salt to taste and serve.