Place the onions in a blender or processor and add 1 cup of the Puree as fine as possible. Peel and chunk in 2 1/2 of the avocados, reserving the remaining half for garnish. Puree again, adding broth as necessary. Transfer the soup to a large bowl. Add any remaining broth the buttermilk to the blender and whir briefly to pick as much avocado as possible. Add to the soup, along with the extract and sugar, if desired. Both of these rather surprising ingredients enhance avocado when used in tiny amounts. Stir well, taste and adjust the salt. The soup may be made to this in advance and refrigerated several hours, but for best flavor allow it to warm up a bit before serving. It should be cold but not icy. Peel the remaining half avocado cut into very thin slices. Divide soup-among six serving bowls and top with the sliced avocado, and coriander. Serve accompanied by the tortilla chips.