Heat a cast iron skillet to medium heat. Put the garlic in and cook until brown on all sides. Put the cooked garlic in a small bowl.

Toast the cumin in the skillet over medium heat until fragrant, 1-2 minutes. Place the cumin in the bowl with the garlic and add the aji panca paste or chipotle peppers. Add the paprika and puree to a paste. Work in the vinegars and enough oil to make a thick paste. Season generously with salt and pepper.

Cut beef hearts into strips about 1 1/2 inches wide. Cut diagonally into strips about 2 inches long, 1 1/2 inches wide, 1/4 inch thick. Weave 3 pieces meat back and forth onto each skewer so that flat side is exposed to the fire.

Put the skewers in a glass baking dish and spread the marinade over the meat with a spoon. Cover and marinate 6 to 12 hours, turning several times.

Heat the grill to high and oil the grill grate.

Drain the marinade off of the kebabs, throwing the marinade out. Arrange on hot grate with aluminum foil under the exposed parts of the skewers to prevent burning. Grill until sizzling, brown, and cooked through (about 10 minutes total). Baste on the grilled side with achiote or olive oil after 3 mins to keep the meat moist (using about 2 tablespoons oil).

Serve hot.