Season pork shoulder with garlic salt and pepper to taste. Place in a baking pot and bake for 30 minutos at 450 degrees F.
In a pot combine tamarind, piloncillo and water. Cover and let it boil at medium-high heat for 20 minutes. Let it cool down for 5 minutes and strain the pulp, you can do this by pushing with a spoon or with your clean hands. You will end with a bunch of big seeds, tamarind threads and big chunks in the colander.
Heat oil in a pan, add garlic, onion and jalapeños. Sauté for 3 minutes. Add vinegar and tamarind pulp. Stir and cook for 3 more minutes. Pour mixture in a food processor and blend until smooth.
After the 30 minutes. Carefully take the pork out of the oven and cover with the tamarind sauce**. Return to oven, lower the temperature to 350 degrees F, and bake for 1.5 more hours or until the interior temperature of the pork reaches between 145-150 degrees F.
Carefully remove from the oven, cover with aluminum foil, and let it rest for 15 minutes. This rest is paramount, please don't skip it. It keeps juices inside and allows the meat to cook completely (mine went from 150 to 160 during the rest).