Most supermarket vanilla extracts are single-strength (a term used to describe the concentration of vanilla flavor) and must be made from 13.35 ounces of vanilla beans per gallon of liquid solvent, according to U.S. Food and Drug Administration standards. Double-strength, or two-fold, vanilla extract, which can be purchased from specialty stores and mail-order spice houses, is adjusted in the final stages of the manufacturing process to increase the "extractive matter" to 26.70 ounces (13.35 doubled).
We purchased a bottle of double-strength vanilla extract from Penzeys Spices and tried it two ways—using half the amount of vanilla the recipe called for in one batch and using the full amount in another—in the following recipes: yellow cupcakes, vanilla frosting, pastry cream, and chocolate chip cookies. As a control, we also made the recipes using our recommended brand of single-strength vanilla, McCormick.
The differences in the uncooked applications—the frosting and pastry cream—were noticeable. While the samples that used the full amount of double-strength vanilla definitely had a more pronounced vanilla flavor, they didn't necessary have a better flavor. In fact, several tasters found them too strong, citing "medicinal" or "alcohol" notes. When we compared single-strength versus halved double-strength vanilla, most tasters preferred the single-strength versions. In the baked goods, the differences were more difficult to detect. Some tasters appreciated the stronger vanilla flavor of the cupcakes that contained the full amount of double-strength vanilla. In contrast, several tasters complained that the double-strength vanilla was "overkill" in the cookies. In general, the single-strength vanilla was again preferred over the halved double-strength vanilla.
So if you happen to have a bottle of double-strength vanilla on hand, use half the amount called for in most recipes, especially those in which the vanilla is stirred in raw. But don't run out and buy a bottle: Even though you only use half as much, a typical 4-ounce bottle costs about three times as much as single-strength supermarket vanilla.