1. Bring a large pot of water to a boil over high heat. Prick sweet potatoes with a fork and boil until tender when pierced, 30 to 35 minutes. Drain.
2. While potatoes are boiling, grate zest from limes and set aside; then squeeze juice from limes and set aside.
3. When potatoes are cool enough to handle, peel and slice into 1/2-in.-thick slices. Arrange on a platter, cover with foil, and put in a 200° oven to keep warm.
4. Melt butter in a small saucepan over medium heat until foaming. Stir in zest and cook until fragrant, 1 minute. Remove from heat and stir in lime juice. Drizzle lime butter over potatoes and sprinkle with salt.
Note: Nutritional analysis is per serving.
Okinawa sweet potatoes, also called purple sweet potatoes, are available at some Asian-food markets, farmers' markets, or online.