Special equipment: a tortilla press; a deep-fry thermometer, optional
Combine the masa, coriander and salt in a medium bowl. Using your hands, cut the duck fat into the masa mixture until it looks like small peas are in the masa and it holds together when you squeeze it. Still mixing with your hands, slowly fold in the water. The dough should be moist but not sticky and hold together as a ball. Cover with a damp paper towel and let rest 10 minutes.
Roll 2 tablespoons of the dough into a ball (the ball should be about the size of a golf ball). Press the ball in a tortilla press lined with wax paper. Hold the pressed tortillas between wax paper and keep any that are exposed to the air covered with damp paper towels.
Heat a cast-iron pan over medium heat. Cook the tortillas in the dry pan until lightly browned and speckled on each side, about 2 minutes per side. Allow to cool completely. Eat immediately, if using as fresh tortillas.
To make tortilla chips: Fill a Dutch oven with about 4 inches of vegetable oil. Heat over medium-high heat until the oil reaches a temperature of 360 degrees F. Cut the tortillas into quarters and, working in batches, fry until golden brown, turning once, 2 to 3 minutes. Season with Lime-Chili Salt if using.
Combine the salt, lime zest and chili powder in a small bowl and use immediately. Great for garnishing cocktails, seasoning tortillas or as a finishing salt for a dish.