Pick through the dried chickpeas, discarding any stones, debris, or discolored beans. Rinse well under cold water.
Place the chickpeas in a large bowl and cover with 4 cups of water. Soak at room temperature for 8 to 12 hours, or overnight. Drain and rinse with fresh water.
Transfer the soaked chickpeas to a medium stainless-steel or ceramic saucepan. Add 5 cups fresh water, the baking soda, and 1/2 teaspoon salt. Bring to a boil, then reduce to a steady simmer.
Cook for 30 to 35 minutes, skimming off foam and loose skins as needed and adding hot water if the pot becomes dry. The chickpeas are ready when they collapse easily when pressed between your fingers.
Drain the chickpeas, reserving 1 to 2 tablespoons for garnish. For an extra-silky hummus, gently rub the warm chickpeas in a bowl of water to loosen skins and skim away the skins.
Add the drained chickpeas to a food processor and process alone for about 1 minute, until they form a smooth, stiff paste.
Add the tahini, lemon juice, chopped garlic, 1 teaspoon salt, and cumin, if using. Process on high for 3 to 5 minutes, stopping to scrape down the sides as needed.
With the processor running, slowly stream in the cold water. Blend until the hummus is pale, fluffy, and creamy. Add additional cold water, 1 tablespoon at a time, if needed, then taste and adjust the salt, lemon juice, or tahini.
Spoon the hummus into a shallow serving bowl and use the back of a spoon to make a swirl and well in the center. Garnish with the reserved chickpeas, olive oil, and a pinch of cumin or sumac. Serve warm or at room temperature.