In a granite mortar, pound the chilies and salt together until you have a smooth paste with no visible pepper seeds.
Add the shrimp paste and continue to pound until smooth.
Add the sliced shallots, a couple of tablespoons at a time, and pound until smooth.
Add the fish meat and pound until you have a smooth paste.
Put the fish stock in a pot and bring it to a gentle boil.
Stir in the prepared paste, followed by tamarind pulp, fish sauce, and palm sugar.
Once the palm sugar completely dissolves, taste and correct the seasoning as necessary. The flavor should be predominantly sour, then salty, and a little sweet.
Your kaeng som curry base is now ready to be used right away or frozen.
To assemble a 2-serving portion of kaeng som, bring 2 cups of the curry base to a boil in a medium pot, add desired vegetables. Once the vegetables are almost ready, add cubes of fish fillets or peeled and deveined shrimp and gently cook until the fish or shrimp is done.
Recipe by SheSimmers at https://shesimmers.com/2011/06/thai-sour-curry-kaeng-som-%e0%b9%81%e0%b8%81%e0%b8%87%e0%b8%aa%e0%b9%89%e0%b8%a1.html