In a large pot, add the potatoes and cover with cold water by about 2 inches. Bring to a boil, reduce to a simmer and cook until tender and easily pierced with a fork, about 15 - 20 minutes. Drain in a colander, and allow them to steam for about 5 minutes. Return them to the pot, or place in a bowl of a stand mixer.
Add the 4 tablespoons of butter, and with a potato masher, mash until smooth or if using a stand mixer with a paddle attachment, mix on low speed until smooth. Season with salt and pepper. Add the cream cheese, milk, egg yolks, and thyme (if using), mix until well combined. Do not over-mash or your potatoes will end up with a gluey consistency.
Preheat oven to 425°F. Line a baking sheet with parchment. You can either use a spoon to dollop the mixture or use a piping bag fitted with a large star point and pipe into equal portions on the baking sheet, about 2 inches apart.
Brush the potatoes with the melted butter, and bake in the preheated 425°F oven for about 18 - 20 minutes or the peaks start to turn golden.
Notes
To save on time, prepare recipe ahead of time, pipe and then refrigerate. Then pull out of the refrigerator to bake in a 425°F oven 20 minutes before serving. I served my Duchess Potatoes with a pile of crispy shallow fried shallot rings. To make, thinly slice shallot. Heat 2 - 3 tablespoons of olive oil in a small frypan until hot. To test if the oil is hot enough, add one small piece of shallot and once it starts to bubble, add in the remaining sliced shallots. Gently stir around occasionally. Fry about 2 minutes or until just starting to turn golden. Remove and drain. They will crisp up as they cool.