Add the potatoes to a pot and add enough cold water to barely cover the potatoes and add the salt. Place over high heat and cover.
Reduce to medium heat once the water comes to a boil and place cauliflower florets on top of the potatoes. Simmer for 15 minutes. Poke the potatoes and cauliflower with a paring knife and if it easily penetrates remove from heat.
Once the potatoes and cauliflower are cooked, remove the cauliflower and drain into a colander and shake to remove excess water. Place the cauliflower into a blender.
Drain the potatoes into a colander and shake to remove excess water. Return to the pot.
Add the warmed milk, melted butter, and sour cream to the blender. Remove the feeder cap and place a clean dish towel over the lid. Puree the mixture until smooth. Taste for salt and pepper.
Set the pot with the potatoes over medium heat to remove any excess moisture from the potatoes. After 2 to 3 minutes, remove from heat.
Using a ricer or potato masher, mash the potatoes until no lumps remain. Add the cauliflower mixture and chives and stir to combine with a spatula. Taste for salt.
Transfer to a large bowl and serve immediately or keep hot in a heat-proof bowl set over simmering water.
Notes
Leftovers can be stored for 3 days in an airtight container.