Place garlic into a non-metallic bowl and whisk in cola, coffee, rice vinegar, tomato sauce, black pepper, 1 teaspoon salt, rosemary and tabasco until combined.
Unroll the skirt steak and cut into 15cm lengths with the grain. Submerge the meat totally in the marinade; place a piece of plastic wrap onto meat and marinade surface. Marinate for 8 to 12 hours in refrigerator.
Preheat barbecue on medium-high heat. Lightly oil the grill plate.
Remove meat from marinade; draining excess marinade into a saucepan. Pat meat thoroughly dry with paper towels. Season both sides of meat with salt and cayenne pepper to taste.
Place marinade over medium heat and simmer until slightly thickened, about 10 minutes. Strain through a fine-mesh sieve into a bowl.
Grill meat on the preheated grill until steak pieces are still pink inside and meat has good grill marks, about 4 minutes per side. Place thin cooked pieces onto thicker pieces of meat to prevent thin ones from overcooking. When the surface of the meat looks shiny and wet from juices being forced up to the surface, the meat is medium-rare to medium in doneness. An instant-read meat thermometer inserted into the centre of a piece should read 52 to 54 degrees C.
Remove meat to a platter and let rest for at least 5 minutes before slicing thinly across the grain to serve. Drizzle servings with reduced coffee-cola marinade.