Crispy Spiced Potato Wedges

Combine potatoes, 1/4 cup salt, and 3 qt. cold water in a 5 –7-qt. pot. Bring to a boil, then reduce to a simmer; cook until just fork-tender, 2 –3 minutes. Drain, arrange in a single layer on a large plate, and chill until cold, about 20 minutes (handle potatoes delicately; they will be fragile until chilled). Pour oil into a wide heavy pot to a depth of 2". Heat over medium-high until deep-fry thermometer registers 350 °F. Meanwhile, coarsely crush 1 Tbsp. mustard seeds. Combine flour, cornstarch, 2 Tbsp. paprika, 1/2 tsp. pepper, 1/4 tsp. garlic powder, 1 tsp. salt, crushed mustard seeds, and remaining whole mustard seeds in a large bowl. Combine buttermilk and chilled potatoes in another large bowl. Dredge potatoes in flour mixture, then transfer to a plate as you go. Working in two batches, fry wedges, turning once halfway through, until crisp, 6 –8 minutes per batch. Drain on paper towels. Transfer to a serving platter, then season with salt, pepper, paprika, and garlic powder.