1/2-1 inch Star Tip for a pastry bag, and a pastry bag. Don't worry if you don't have it, there are other ways to cook these babies!
Potato ricer, or masher
2-3 Large russet potatoes (Starchy ones)
1 Egg yolk, lightly beaten
Salt and Pepper
2 Sprigs fresh rosemary, leaves only finely chopped, or about 1 Tablespoon dried
2 Tablespoons Milk
2 Tablespoons butter, melted
Bake or boil the potatoes until cooked through and soft. Once soft and cooked, peel the potatoes if you hadn't already, and finely mash them or put through the potato ricer. To the mashed potatoes add the rosemary and milk, season with salt and pepper and gently fold in the egg yolk until the potatoes are smooth and creamy.
Now if you are lucky enough to have a pastry bag and a large star tip, fill the pastry bag with the potato mixture, and pipe the potatoes onto a pan lined with parchment paper. You want to create little potato "star puffs" that are about 3 inches in diameter and about an inch or two high. Just start by making a 3 inch diameter circle and then fill it in, then add another layer or two on top of the first one to give the puffs some height. Once all the potato has been shaped, brush the tops with the melted butter and baked for about 20 minutes at 375°F, or until nicely toasted golden brown. Serve right away.
If you do not have a piping bag and large star tip, the potato mixture can be smoothed into a casserole dish. Then using the tines of a fork, score and rough up the top surface. Coat the top with butter and then bake for 20 minutes at 375°F.
Finally I also have in my possession some festive cookie presses, that are basically just a stamp to emboss a design or shape into the tops of cookies which are rolled into balls and then pressed flat. Sometimes to be creative I will roll the potato mixture into balls and using the cookie presses sprayed with Pam cooking spray press a design into the tops of the balls. Then brush with butter and bake.
Because mashed potatoes can only be eaten so many times before a change is needed. Duchess potatoes are basically just an extra fancy mashed potato, with egg, cream, herbs and butter added for flavor. They could also easily be described as twice-cooked mashed potatoes.
The wonderful thing about Duchess potatoes is that they can easily be made ahead and then toasted to warm right before serving. If you are planning to make these ahead of time all you need to do is follow all the instructions until just before the end. Wait until you are actually going to do the final toasting before brushing the rounds with butter.