Position racks in the middle and upper third of the oven; preheat to 400 degrees F. Scrub the potatoes and dry well. Bake directly on the middle oven rack until easily pierced with a knife, 45 to 50 minutes. Remove from the oven; let cool 5 minutes.
Meanwhile, melt 3 tablespoons butter in a large skillet over medium-high heat. Add the spinach (in batches, if needed) and cook, tossing with tongs, until wilted, about 5 minutes. Add the garlic, nutmeg, cayenne and 1/2 teaspoon salt. Increase the heat to high and cook until the liquid evaporates, about 2 minutes. Transfer the mixture to a cutting board and finely chop; squeeze dry in a kitchen towel.
One at a time, hold each potato with another kitchen towel and halve lengthwise. Scoop the flesh into a large bowl, leaving a 1/4-inch-thick shell. Mash the flesh with the remaining 3 tablespoons butter until smooth; season with salt. Stir in the egg yolks and parmesan, then the spinach mixture. Brush the potato skins with melted butter; season the insides with salt and pepper. Set on a baking sheet.
Beat the heavy cream in a large bowl with a mixer on medium speed until soft peaks form, about 2 minutes. Stir about one-quarter of the whipped cream into the potato mixture, then gently fold in the rest.
Mound the filling into the potato skins. Return to the oven on the top rack and bake until the filling is warmed through and starts browning, about 15 minutes. Turn on the broiler and broil until the filling is golden brown and the skins are crisp, 1 to 2 more minutes.
Photograph by Ryan Liebe
Courtesy of Food Network Magazine