Prepare the potatoes by peeling and quartering them. Then, add them to a large pot and submerge them in water. Salt the water. Heat the pot over high heat and bring to a boil, cook for 15-20 minutes or until the potatoes are fork-tender.
Preheat the oven to 425℉ and prepare a baking sheet with parchment paper.
When the potatoes are done, drain them and transfer them into a bowl. Mash the potatoes with a potato masher until the mixture is smooth and the lumps are gone.
Stir in 3 tablespoons of butter, mix until melted. Add in the egg yolks one at a time. Then, add the remaining ingredients into the potatoes and combine.
Insert a star tip into a piping bag and then fill it with the mashed potatoes.
Create the duchess potatoes by piping ice cream-shaped mounds onto the baking sheet. You should be able to get around 12 mounds.
Heat the remaining 2 tablespoons in a microwave safe bowl. Then, brush the melted butter onto the potato mounds.
Bake at 425ºF for 18-20 minutes.
When the potatoes are done, let them rest for 5 minutes before serving.