The evening before you want to make your hummus place the dried garbanzo beans in a large bowl and cover with water two to three times their volume.The next day drain the garbanzo beans and rinse well.
In a medium saucepan over high heat combine the garbanzo beans and the baking soda, stirring constantly for 3 minutes.
Add 6 1/2 cups water to the pot and bring the beans to a boil. Continue to cook the beans over medium-high to high heat for anywhere from 20 -40 minutes depending on freshness. Skim off any foam or skins that float to the surface during the cooking process with a slotted spoon and discard them. When the beans are tender enough to break up easily, but not turn to mush when pinched between your thumb and finger they are done. Drain the cooking liquid.
Transfer the cooked garbanzo beans to a food processor fitted with the "S" blade and process until a thick yellowish paste forms.
With the machine running, add the homemade tahini, lemon juice, garlic, and salt. Once they have been incorporated drizzle in the ice-cold water and continue to process for 5 minutes until the hummus is smooth and velvety.
Transfer the hummus to a bowl and cover the surface with plastic wrap so that there is no way that air can get in between the hummus and the plastic wrap. Let the hummus sit for 30 minutes at room temperature, during this time the hummus will thicken slightly.