Step 1In a non-reactive pot or container, combine the pumpkin seeds, water, allspice and cinnamon. Cover and steep the mixture for 2 hours on the countertop, or refrigerate overnight. (If you’re pressed for time, soak the ingredients using hot water and store at room temperature for 90 minutes; this will yield a slightly less-creamy horchata.)
Step 2Pour the mixture into a blender (this may need to be done in batches), along with the sugar and salt and blend at high speed until the seeds are completely broken down and the mixture is light and creamy, 3 to 4 minutes.
Step 3Strain the mixture using a fine mesh strainer, pushing out all of the liquid and discarding the solids. Add additional water to thin if desired. This makes about 1 ½ quarts horchata, depending on your desired thickness. The horchata will keep, covered and refrigerated, up to 2 to 3 days. Serve over ice.