Cut potatoes into 1’’ slices, cut width-wise. Place in a large pot of water with 1/4 c salt, and bring to a simmer over medium-low heat. Simmer your potatoes for 8-10 minutes; they will be easily poked through with a fork.
When potatoes can be poked through with a utensil, drain them and place them back in the pot. Let them sit for about 5 minutes, and allow the extra water to evaporate.
Next, add butter, salt, pepper, egg yolk, and cheeses, and mash with a potato masher or a fork until well incorporated and smooth. Finally, add in chives. Serve warm and enjoy!