Preheat the oven to 450°. Heat the vegetable oil in a large skillet. Add the ground lamb, cumin, cocoa powder, chile powder and crushed red pepper and cook over moderate heat, stirring to break up the ground lamb, until the meat is cooked through, about 6 minutes. Stir in the onion, season with salt and pepper and transfer the lamb to a bowl. Add the refried beans to the skillet and cook over moderate heat until they are hot.
Lay the tortillas on a work surface and spoon one-quarter of the ground lamb in a band across the center of each tortilla, leaving a 2-inch border on either side. Top each with one-quarter of the refried beans, cheese, chopped tomato, cilantro and salsa and roll up the tortillas, folding in the ends as you go. Put the rolled tortillas in a large baking dish and bake them for about 10 minutes, or until hot and bubbling. Serve the burritos at once, passing the sour cream at the table.