These crispy cakes will hold in the oven for a few minutes, but serve them as soon as possible for maximum deliciousness.
Preheat oven to 175°F. Grate sweet potato using large holes of a box grater to equal 4 cups. Grate onion using large holes to equal 1/4 cup. (Discard remaining potato and onion, or reserve for another use.) Working in batches, place half of the grated potato in center of a kitchen towel; squeeze over a sink to remove as much liquid as possible. Transfer squeezed potato to a medium bowl. Repeat process with remaining grated potato and onion.
Add flour and cornstarch to potato mixture; stir well to coat. Add eggs, salt, cumin, and chile powder; stir well to coat. Stir together yogurt and lime juice in a small bowl; set aside.
Heat one-third of the oil in a large nonstick skillet over medium-high. Spoon 4 (1/4-cup) portions of potato batter onto skillet, flattening each portion using a spatula into a loose 3-inch disk. Cook until bottoms are crisp, about 4 minutes. Flip; cook until other sides are crisp, 3 to 4 minutes. Drain on a plate lined with paper towels 1 minute; transfer to a baking sheet, and keep warm in preheated oven. Repeat process twice using remaining oil and potato batter.
Arrange warm latkes on a serving platter. Top each latke with about 1 1/2 teaspoons yogurt-lime sauce; sprinkle evenly with pumpkin seeds and chives.