Step 1

Place potatoes in a large pot and cover with cold water by 1 inch. Generously salt water and bring to a boil. Reduce heat to medium-low; cook, stirring occasionally, until potatoes are very tender but not falling apart, 20 to 30 minutes. Drain. Press with a ricer and return to pot, or mash with a potato masher.

Step 2

Meanwhile, melt ¼ cup butter in a medium skillet over medium. Add leek and ½ teaspoon salt. Reduce heat to medium-low; cook, stirring occasionally, until leek is very tender, 10 to 15 minutes. Add garlic; cook, stirring constantly, until fragrant, about 1 minute.

Step 3

Add sour cream, half-and-half, leek mixture, pepper, and remaining 2 teaspoons salt to potatoes. Stir well to combine.

Step 4

To Serve Immediately: Transfer to a serving dish. Top with remaining 1 tablespoon butter and more pepper