Adjust oven racks to lowest & top positions & heat oven to 450 deg.

Combine ¾ t salt, chili powder, paprika, pepper, garlic powder & cayenne in small bowl.

Bring 1 qt water to boil in pot.

Place 1 thigh skin-side down on cutting board. Cut away any pockets of fat & any skin that extends beyond meat & reserve trim. Make three ½"-deep slits in flesh parallel to bone. Using tip of paring knife, poke skin of thigh 8-10 times. Repeat entire process with remaining thighs.

Sprinkle ¾ t spice mixture evenly over skin of thighs. Turn thighs over & sprinkle remaining spice mixture over flesh side. Rub mixture into slits.

Scatter trim over surface of rimmed baking sheet. Roast on lower rack until trim is mostly crisped & fat is rendered, 8-10 min. Discard trim & leave fat in baking sheet.

While trim is rendering, add potatoes & baking soda to boiling water & return to boil. Boil for 2 min & drain well. Return potatoes to pot & place over low heat. Cook, shaking pot occasionally, until any surface moisture has evaporated, about 2 min. Remove from heat.

Add rendered fat & remaining 1 t salt to potatoes & stir with rubber spatula until potatoes are coated with starchy paste, about 30 sec. Transfer potatoes to now-empty baking sheet & spread into even layer. Roast on lower rack until undersides of potatoes are brown & crisp, about 15 min.

Flip potatoes & then push them aside to clear a space for each thigh. Place thighs skin side up in cleared spaces. Roast on upper rack until chicken is browned & crisp & thighs register at least 185 deg, 25-30 min.

Serve immediately with lime wedges.