Boiling our Duchess Potatoes made them waterlogged. Next we tried baking them, but they quickly dried out. We solved this problem by parcooking the potatoes in the microwave and finishing them in the oven on high heat. Piling butter into the hot potato mixture made it too soft to pipe into delicate mounds. We added some while it was still hot, but waited to add the rest until the potatoes had cooled a bit. The result was rich, buttery flavor throughout with small butter pockets scattered in the mounds. For the smoothest, most uniform texture, use a food mill or ricer to mash the potatoes.
1. MICROWAVE POTATOES Adjust oven rack to upper-middle position and heat oven to 475 degrees. Meanwhile, prick potatoes all over with fork, place on plate, and microwave until tender, 18 to 25 minutes, turning potatoes over after 10 minutes.
2. MASH POTATOES Cut potatoes in half. When cool enough to handle, scoop flesh into large bowl and mash until no lumps remain. Add cream, 3 tablespoons butter, eggs, salt, pepper, nutmeg, and baking powder and continue to mash until potatoes are smooth. Let cool to room temperature, about 10 minutes. Gently fold in remaining butter until pieces are evenly distributed.
3. PIPE AND BAKE Transfer potato mixture to piping bag fitted with ½ -inch star tip. Pipe eight 4-inch-wide mounds of potato on rimmed baking sheet. Spray lightly with cooking spray and bake until golden brown, 15 to 20 minutes. Serve.
MAKE AHEAD: Once piped onto the baking sheet, the potatoes can be covered loosely with plastic wrap and refrigerated for 24 hours. Remove plastic and spray lightly with cooking spray before baking.