Place the flank steak in a baking dish or wide bowl. Sprinkle all over with brown sugar, onion powder, chili powder, cumin, 1 teaspoon salt, and black pepper. Drizzle with olive oil, rub oil and spices into the meat.
Pour porter, Worcestershire, soy and garlic over the steak. Cover and refrigerate for 2 hours.
Remove steak from marinade, cut into 2 inch strips, thread through metal skewers (or pre-soaked wooden skewers). Sprinkle with remaining salt.
Grill until desired doneness, about 3 minutes per side for medium rare (depending on thickness.
In a small bowl stir together the sour cream, horseradish, cilantro, ¼ tsp salt, lemon juice, and garlic powder (can be made up to a day ahead of time, flavors develop after a few hours).
Serve skewers with sauce on the side.