The night before, put the chickpeas in a large bowl and cover them with cold water at least twice their volume. Leave to soak overnight.
The next day, drain the chickpeas. Place a medium saucepan over high heat and add the drained chickpeas and baking soda. Cook for about 3 minutes, stirring constantly. Add the water and bring to a boil. cook, skimming off any foam and any skins that float to the surface. The chickpeas will need to cook between 20 and 40 minutes, depending on the type and freshness, sometimes even longer. Once done, they should be very tender breaking up easily when pressed between your thumb and finger, almost but not quite mushy.
Drain the chickpeas. You should have roughly 3 2/3 cups now. Place the chickpeas in a food processor and process until you get a stiff paste. Then, with the machine still running, add the tahini paste, lemon juice, garlic, and 1.5 tsp salt. Finally, slowly drizzle in the iced water and allow it to mix for about 5 minutes, until you get a very smooth and creamy paste.
Transfer the hummus to a bowl, cover the surface with plastic wrap, and let it rest for at least 30 minutes. If not using straightaway, refrigerate until needed. Make sure to take it out of the fridge at least 30 minutes before serving.