Pierce potatoes all over with a fork, and place on lined baking sheet. Bake until tender, about 50-60 monutes depending on size.
When cool enough to handle, slice potatoes in half length wise. Scoop potato pulp into a medium bowl with ginger paste, milk, butter, kosher salt, cinnamon, nutmeg, and pepper. Mash with a potato masher until smooth. Spoon mashed potato mixture evenly into potato shells. Place back on baking sheet.
Meringue
Beat egg whites, cream of tartar, and kosher salt with a mixer on high speed until foamy, about 1 minute or less. Carefully add the powdered sugar, a tablespoon at a time, again beating at high speed, about 2 minutes or until stiff peaks form.
Spoon meringue evenly over potatoes. Dab quickly with fingertip to make meringue form small peaks (see photos). Bake until meringue is golden brown, 7 to 8 minutes. Watch carefully so they don't get too browned!
Notes
Fresh ginger and or paste is preferred, but a substitute can be 1/4 teaspoon of ground ginger.
You can also substitute half and half or heavy cream for the milk.
If you don't have and cream of tartar on hand, substitute same amount of white vinegar. It helps stabilize the meringue.