Basic Pommes Duchesse
1-2 pounds potatoes (I find mealy potatoes such as Russet work best)
1-2 egg yolks, at room temperature
~ 1/2 cup butter (more or less), salted or unsalted just adjust salt content accordingly
salt and ground black pepper, to taste
other dry spices, as desired
chopped fresh or dried herbs, as desired
Peel and wash the potatoes and cut into bite-sized cubes. Boil until very tender, then drain well. Let stand for a few minutes to dry out, then mash. I usually just mash by hand, but if you have a large amount or want them very smooth, use a potato ricer.
Cut the butter into cubes and add along with the egg, and seasonings, and mash them in until completely smooth.
Transfer to a piping bag. I always use one fitted with a star tip - either a small one to make small rosettes or a larger on to make big rosettes.
Pipe potatoes on a baking sheet lined with parchment paper. They can be refrigerated at this point of baked right away at 425◦F until golden on top and firm - larger rosettes will take longer.
A few tips and tricks: Potatoes should be very smooth and herbs should be cut fine so they can be piped. Potatoes need to be piped while they are still hot. Ensure the butter is in cubes and not melted so it incorporated without making the potatoes too liquidy. If tops do not color enough, broil them for a few minutes.