How to Make It
Melt the butter in a large saucepan over medium. Add the garlic and cook, stirring often, until fragrant, 2 to 3 minutes. Add the half-and-half and salt and gently simmer 5 minutes. Remove from heat.
Place the Yukon Gold potatoes in a large pot with salted water to cover and bring to a simmer over medium-high. Increase heat to high; add the russet potatoes and bring to a boil. Reduce heat to medium-low and simmer until all the potatoes are tender, 10 to 12 minutes. Drain and return to pot.
Add the half-and-half mixture and sour cream to the potatoes; mash to the desired consistency. Top with the parsley and pepper.