Stem the chiles, and depending upon taste, keep or discard all or a portion of the seeds (which will make the paste hotter). Place in a bowl and add enough warm water to cover. Soak for 30 minutes. Drain, reserving the water.
Heat a small skillet over medium heat. When hot, add the shallots, ginger, and coriander seed and cook, stirring constantly, until aromatic, 2 to 3 minutes. Add the garlic and cook, stirring, for 30 seconds. Remove from the heat.
Combine all the ingredients in a blender or food processor. Process into a moist paste, stopping frequently and scraping down the sides, and adding 1 tablespoon of the chili soaking liquid or extra oil as necessary to make moist. Adjust seasoning, to taste.
Place in an airtight container and refrigerate until ready to use. (Paste can be stored in the refrigerator for up to 1 month or in the freezer for up to 3 months.)
Yield: about 1 1/4 cups