Instructions:

1. Cook the Potatoes: Place the scrubbed potatoes in a large pot and cover with cold water by about an inch. Add a generous amount of kosher salt to the water. Bring to a boil over high heat, then reduce to a simmer. Cook until the potatoes are very tender but not falling apart, about 30–35 minutes. Drain the potatoes, briefly rinse with cool water to remove excess starch, and return them to the warm pot off the heat to dry.​
2. Infuse the Dairy: In a small saucepan, combine the milk, heavy cream, halved garlic head, and rosemary sprigs. Warm over medium heat until fragrant, about 5 minutes. Remove from heat and let it steep while you prepare the potatoes.​
3. Mash the Potatoes: Pass the hot, dried potatoes through a potato ricer or food mill into a large bowl. Add the room-temperature butter pieces and the measured kosher salt. Stir until the butter is fully incorporated and the mixture is smooth.​
4. Incorporate the Infused Dairy: Strain the warm milk mixture through a fine-mesh sieve into a measuring cup, discarding the solids. Gradually add the infused milk to the mashed potatoes, about ½ cup at a time, stirring after each addition until fully incorporated and the desired creamy consistency is achieved.​
5. Season and Serve: Taste and adjust seasoning with additional salt if needed. Top with freshly ground black pepper and serve warm.

For more details and tips, you can visit the original recipe here