Place chicken stock in a medium saucepan over high heat and reduce to 4 cups. Heat olive oil in a medium saucepan over medium heat. Add the onions and garlic and cook until soft. Add the chile powders and cook for 2 minutes. Add the reduced stock and bring to a simmer, whisk in the tamarind paste and brown sugar. Add the pineapple and cook over medium-low heat until the pineapple is very soft, 30 to 40 minutes. Place the mixture, in batches, in a blender and strain into a clean saucepan. Cook for 15 to 20 minutes until reduced slightly. Season with salt and pepper to taste.
Preheat oven to 400 degrees F. Heat olive oil in a medium saute pan over high heat. Season the pork with the ancho powder, salt and coarsely ground black pepper. Sear well on all sides until golden brown. Place pan in the oven and continue cooking for 8 to 10 minutes until the pork is just cooked through. Remove and let rest 10 minutes before slicing on bias. Spoon sauce onto platter, top with pork slices, drizzle with sauce.
Buttermilk-Cascabel Chile Onion Rings:
In a mixing bowl, combine the flour, buttermilk, cayenne and salt to taste to make a batter. Soak the onion rings in the batter for 2 hours or longer in the refrigerator. When ready to fry, heat the oil to 375 degrees F, or until a drop of batter sizzles. Dredge the onion rings in seasoned flour and fry, turning, in batches, until tender and golden brown, 3 to 4 minutes. Drain on paper towels and season with salt. Serve immediately.