This recipe is from True Thai, by Victor Sodsook (see review by Adam Hiller)

Chicken Coconut Soup
with Siamese Ginger and Lemon Grass
(Tom Kha Kai)

Put the stock, ginger, and lemon grass in a soup pot.  It using Kaffir lime leaves, tear each leaf in half and add to the pot.  If using lime peel, add to the pot.  Gradually bring the stock to a boil over medium high heat.  Boil for 1 minute, stir in the coconut milk, and return to ta boil.  Stir in the chicken and return to a boil.  Add the chili tamarind paste, lemon juice, sugar, and fish sauce.  Stir until the chili tamarind paste and sugar are dissolved and blended.  Add the mushrooms and simmer just until tender, about 1 minute.

Float the chilies on top and turn off the heat.  Ladle the soup into a steamboat, a soup tureen , or individual serving bowls.

Note: Frequent customers of Thai restaurants know that the Siamese ginger, lemon grass, and lime that flavor Thai soups are not meant to be eaten.  Eating the chilies is optional, but not recommended if you're not into "heat."

Chili-Tamarind Paste:

Put dried shrimp in a small bowl and add water to cover.  Swish around a few times for a brief rinsing, then drain.  Set aside.

Pour 1 1/2 cups o the oil into a wok or heavy saucepan set over medium high heat and bring the oil to 360 degrees Fahrenheit.

Add the garlic and cook until golden brown, about 1 minute, stirring occasionally.  Remove with a wire skimmer or slotted spoon to a bowl lined with paper towels.  Return the oil temperature to 360 degrees Fahrenheit before adding each of the next three ingredients.

Fry the shallots, stirring often, until they begin to brown, about 2 - 3 minutes.  Add the to the bowl with the fried garlic.  Fry the shrimp, stirring occasionally, for 1 minute.  Add them to the bowl.  Fry the chilies until they darken and become brittle, about 30 seconds.  Add them to the bowl of fried ingredients.

Transfer the fried ingredients to a blender or the bowl of a food processor fitted with a metal blade.  Add the remaining 1/4 cup of oil and the tamarind sauce and blend until a moist, nearly smooth paste forms.  Stop occasionally to scrape down the sides of the blender or work bowl.

Transfer the chili paste to a small, deep skillet or saucepan and set over medium heat.  Add the sugar and fish sauce and cook, stirring occasionally, until the color deepens to a rich dark brown, about 50 minutes.  Remove from the heat and cool to room temperature.

Transfer the chili paste to a glass jar with a tight fitting lid.

This paste with keep for about 3 months if stored in the refrigerator.

Tamarind Sauce Recipe:

Put the tamarind pulp in a medium bowl and add the warm water.  Soak until soft, about 20 minutes.  Break the pulp apart with your fingers, then mash and stir with a wooden spoon to help it dissolve.

Pour the mixture through a fine mesh stainer into another bowl.  Stir and mash the soft pulp against the strainer to extract all the thick sauce.  Scrape the sauce clinging to the bottom of the strainer into the bowl.

Discard the fibrous pulp collected in the strainer.

This sauce, if refrigerated in ta tightly sealed container, keeps indefinitely.  For best flavor, however, use it within 1 week.