In a large bowl, combine chickpeas, cumin, cornstarch, 1/4 c tahini, 1/4 c parsley, onion, and 1/4 c yogurt. Mix to combine, then using a large fork or potato masher, smash the mixture until it breaks down into coarse paste with only a few whole chickpeas remaining.
Form the mixture into golf ball-sized rounds and gently press down to create patties.
In a large skillet, heat the oil until it sizzles when you drop in a bit of the chickpea mixture. Gently and carefully arrange the patties in the skillet and cook 5-8 minutes per side, or until deep golden brown and crispy.
While the patties fry, combine the remaining yogurt, tahini, and parsley, then season with salt and pepper to taste.
Remove the falafel patties from the heat and place on paper towels to remove excess oil.
To serve, arrange patties on a platter with a bowl of the tahini-yogurt sauce for dipping. Garnish with herbs if you have an extra lying around, and enjoy!