Combine all of the ingredients in a high-powered blender and blend for 45-60 seconds or until very smooth, scraping down the sides a few times as needed.
The dressing can be used right away but the flavor will improve over the first day or so.It will keep in the refrigerator for several weeks as the vinegar and oil preserve the basil, but the flavor is freshest over the first week. Once the mixture has been refrigerated for a day or so, it will develop a nice thickness. Let sit at room temperature for a few minutes and stir before using.