Place the chickpeas in a medium bowl and cover with 2 inches of cold water. Soak overnight.
Place the cumin and coriander seeds in an 8-inch cast-iron skillet and set over medium high heat. Cook, shaking the pan frequently, until the seeds give off an aroma and just begin to brown, 3 to 4 minutes. Transfer the toasted spices to a spice grinder and process until finely ground. Set aside.
Drain the chickpeas. Combine the soaked chickpeas, ground cumin and coriander, garlic and scallions, salt, black pepper, baking powder, cayenne pepper, and parsley leaves in a medium mixing bowl.
Pass this mixture through a meat grinder or stand mixer with the grinder attachment fitted with the smallest die. Alternatively, you can divide the mixture in half and process one half at a time in a food processor, pulsing 10 to 20 times.
Scoop the mixture into 1 1/2 to 2-ounce portions using a 2-inch diameter disher. Place on a half sheet pan lined with parchment. Can be held hold at room temperature for up to 2 hours before frying, or covered in the refrigerator overnight.
Heat the peanut oil in a 5-quart Dutch oven over high heat until it reaches 350 degrees F. Adjust the heat to maintain temperature. Gently place the falafel, 1 at a time, into the hot oil. Fry 4 at a time until deep golden brown, 5 to 7 minutes. Remove to a half sheet pan lined with a cooling rack and topped with a paper towel to drain. Repeat until all falafel have been cooked. Serve on warm pita.
Recipe courtesy of Alton Brown, 2010