1/2 cup hot water
1+1/2 Tbsp. tamarind paste (available by the jar at Asian/Indian food stores)
5 Tbsp. fish sauce (available in tall bottles at Asian stores) OR substitute 6 Tbsp. soy sauce if vegetarian
1 fresh red chili, minced, OR 1/2 to 1 tsp. cayenne pepper, OR 2-4 tsp. chili sauce
5-6 Tbsp. brown sugar
1. Pour the hot water into a small mixing bowl or glass measuring cup. Add the tamarind paste and stir well to dissolve.
2. Add the other ingredients and stir to dissolve. (Note that you will need all the sugar to balance out the intense sourness of the tamarind.)
3. The sauce should taste tangy - a balance of sour, sweet, spicy, and salty. If it's still too sour for your taste, add a little more sugar. Remember that once it's distributed throughout the noodles, the sauce will no longer be so spicy or strong-tasting.
Yield: 2/3 cup Pad Thai Sauce which is enough for 2 batches of pad thai. Store in a jar in the refrigerator. The sauce will keep for several weeks.
Recipe modified from About.com