Overnight soak: Place the dry chickpeas in a large bowl, add enough water to cover them by 1 inch, and let sit uncovered at room temperature for 20 to 24 hours. They should double in size to give you about 3 cups of chickpeas. When ready, you should be able to break apart a chickpea with your fingers. Drain.
Quick soak: Place the chickpeas and 1 quart of water in a 4-quart saucepan. Bring to a boil over medium-high heat and cook for 3 minutes. Remove the pan from the heat, cover, and set aside for 1 hour. Drain.
Mix: Place the drained chickpeas, scallions, garlic, parsley, olive oil, lemon juice, cumin, coriander, and salt in a large bowl and stir to combine.
Process: Transfer the chickpea mixture to a food processor fitted with the blade attachment, working in batches if needed. Pulse each batch until the chickpeas are finely chopped, 10 or 12 pulses.
Add the baking powder: Sprinkle the baking powder over the mixture. Pulse the chickpeas again, scraping down the sides of the bowl as needed, until the mixture forms a ball when you squeeze it in your hand.
Shape the falafel: Scoop the falafel into 2 tablespoon-sized balls (a small ice cream scoop works well for this) and place on a parchment paper-lined baking sheet.
Heat the oil: Heat the oil in a 10-inch straight-sided skillet over medium-high heat until it reaches 350°F. (Pan-frying is typically too shallow to get an accurate temperature reading — a clip-on thermometer works well for this.)
Fry: Gently lower 6 falafel at a time into the hot oil and fry until deep golden-brown, flipping them halfway through, 5 to 7 minutes total per batch. Remove the cooked falafel to a paper towel-lined baking sheet.
Serve: Serve the cooked falafel in a warmed pita with yogurt or tzatziki sauce, tomatoes, feta cheese, and shredded lettuce.
Notes
Storage: Falafel can be made up to 1 day in advance, and stored in a single layer in an airtight container in the fridge.