Place raw Yukon Gold potato cubes in a large steamer basket over a large pot of boiling water. Cover and steam until very tender, 25 to 30 minutes.
When potatoes are almost tender, melt butter in a small saucepan. Add garlic; cook and stir until fragrant, about 1 minute (or longer for a more mellow garlic flavor, but don't let it brown). Add whole milk and salt; stir until just heated through. Remove from heat.
Press hot potatoes through a food mill or ricer into a large bowl (recipe note #3). Pour butter mixture over and add sour cream; stir until well combined. Garnish with chives if desired and serve.
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Notes
If you don't have a steamer or want to make more mashed potatoes at once, you can boil the potatoes instead. Directions: To a large pot, add cubed potatoes and enough cold water to cover by 1-inch. Bring to boil over high heat. Reduce heat and let summer until potatoes are tender. Drain and proceed with recipe.
If you're serving these for a holiday meal, and need more servings, you can double or even triple this recipe. Click on the Servings number near the top of this recipe card and slide it to the right until you reach the number of servings you'd like, the ingredient amounts will adjust accordingly.
If you don't have a ricer or a food mill, use a potato masher to mash potatoes to your desired consistency before adding butter mixture and sour cream.