So many flavors, so little Dexatrim. First there are the standards: chocolate, malted vanilla, strawberry. Then come the more esoteric mixes, such as amaretto, carob, and Dutch orange chocolate. They’re each the base for Boston’s zenith of all things sweet, cold, and frothy: the Herrell’s frappe. (A little schooling for confused out-of-towners: In New England, a milk shake is milk and syrup, without ice cream. A frappe is a blend of all three.) And what texture, dense enough to make you work at drawing the first sip through the straw, hit with just the right ice-cream-to-milk ratio, and icy enough to hold up on the hottest day. What else did you expect from local ice cream bodhisattva Steve Herrell?