Place the dried chickpeas in a large bowl and cover them with several inches of cool water, since they will roughly double in size. Let them soak at room temperature for at least 12 hours and up to 24 hours.
Drain the chickpeas well and pat them mostly dry with a clean towel before moving on. Do not use canned chickpeas here, since they carry too much moisture and will make the mixture fall apart.
Add the roughly chopped onion and peeled garlic cloves to the bowl of a food processor. Pulse several times until the onion and garlic are finely chopped, scraping down the sides as needed.
Add the parsley and cilantro and pulse again until the herbs are finely chopped and evenly distributed.
Add the drained chickpeas, cumin, coriander, cayenne if using, salt, black pepper, and baking powder. Pulse in short bursts until the mixture is coarse and holds together when pressed, similar to wet sand. Avoid over-processing into a smooth paste, since this will make dense, heavy falafel.
Transfer the mixture to a bowl, cover it, and refrigerate for at least 30 minutes and up to a few hours. This resting time lets the baking powder activate and firms up the mixture so it holds its shape better in the hot oil.
Scoop the chilled mixture using a small cookie scoop or two spoons, using about 2 tablespoons per piece. Roll each portion into a ball, or gently flatten into a small patty if you prefer that shape.
Press a small piece together in your hand first to confirm it holds shape well before shaping the full batch.
Pour about 2 to 3 inches of neutral oil into a heavy bottomed pot or Dutch oven. Heat the oil over medium heat until a deep fry thermometer reads 350 degrees Fahrenheit. Keep an eye on the temperature throughout frying, since it will naturally rise and fall as you add falafel.
Carefully lower a few falafel into the hot oil at a time, being careful not to crowd the pot. Fry for 3 to 4 minutes, turning occasionally, until they are deep golden brown on all sides. Maintain the oil temperature between batches, letting it come back up to 350 degrees Fahrenheit before adding more falafel.
Use a slotted spoon or spider strainer to transfer the fried falafel to a paper towel lined baking sheet. Let them rest for a minute or two so any excess oil can drain off. Serve warm, ideally within the first 20 minutes for the crispiest texture.