Special equipment: a mandoline (optional); a deep-frying thermometer
In a heavy Dutch oven, heat 3 to 4 inches of oil to 320 degrees F (use a thermometer to keep the temp right!).
Trim the tops off the sweet potatoes and finely slice into rounds using a mandoline or knife. Pat them dry with kitchen paper.
Mix together the garlic salt, black pepper, paprika, chipotle, cayenne and kosher salt in a small bowl and set aside.
Lower a batch of potatoes into the oil with a spatula. Cook, stirring frequently, until crispy, 3 to 4 minutes. Remove with a slotted spoon or spider to a baking sheet lined with parchment. Sprinkle on some of the spice mix. Cook the remaining batch.