Cover the chickpeas with water in a medium bowl. Set aside at room temperature to soak for at least 12 or up to 24 hours. They will end up doubling in size.
Drain the chickpeas. Rinse and dry them as thoroughly as possible — I like to use a salad spinner for this.
Place the chickpeas and salt in the bowl of a food processor. Pulse to fine, pebble-sized pieces. Add the parsley, cilantro, onion, garlic, cumin, coriander, and black pepper. Blend until the mixture turns into a thick paste, scraping down the sides as needed. Remove the blade from the bowl, then cover the bowl of the food processor and refrigerate for 1 hour (this lets allows the batter to set).
Remove the falafel batter from the fridge. Sprinkle the baking powder over the top, then gently fold it throughout.
Line a baking sheet with parchment paper and another baking sheet with a double layer of paper towels. Set aside.
Using a medium spring-loaded cookie scoop (or your hands), scoop, firmly pack, and roll about 1 ½ tablespoons of batter into a round ball and add it to the parchment-lined baking sheet. Repeat with the remaining batter. Don’t worry if the balls feel a little loose at first — they will bind once they fry.
Heat 1 inch of oil in a large, high-sided frying pan over medium heat. When a small piece of batter dropped into the oil begins to sizzle, carefully add 10 to 12 of the balls, being careful not to crowd the pan. Fry until golden-brown and crispy, 3 to 3 ½ minutes per side. Using a slotted spoon, transfer the cooked falafel to the paper-towel-lined baking sheet. Repeat with the remaining uncooked falafel.
Transfer the falafel to a serving platter alongside the tahini sauce for dipping. Serve warm.