Ancho chile peppers are poblano peppers that have been dried. Each has its own flavor, one quite different from the other. In different parts of Mexico, where the ancho chile is used profusely,cooks have different opinions about how to treat it. Some like to toast it, some fry it, and some rehydrate it, as described below. The real question, though, is its condition before you soak it in hot water. We find that the most important factor is the quality of the ancho chile you buy. It should be new, plump, and shining, even when dried. Chiles that are old and very dry do not have full flavor.
 Serve this mayonnaise with grilled swordfish, cold roast chicken, or cold roast beef.