Heat the chickpeas, onion, cumin and stock until the stock boils. Reduce the heat and simmer for 20 minutes. Puree the mixture in a food processor and return to the pot. Mix in the flour and 2 tablespoons of butter to a paste and add it to the soup in small pinches. After each addition whisk until smooth. Simmer the soup for 10 minutes. Add the remaining butter and enough half and half to reach the desired consistency. Correct seasoning and garnish generously with cilantro.