These should give you an idea:
Cakes, American muffin, chocolate, 30g each 6
Cakes, American muffin, chocolate, 70g each 14
Cakes, American muffin, chocolate, 110g each 21½
Cakes, American muffin, chocolate chip, 30g each 6½
Cakes, American muffin, chocolate chip, 70g each 14½
Cakes, American muffin, chocolate chip, 110g each 23
Starbucks, Chocolate Muffin with Belgian Chocolate Sauce Centre, each 22½
Marks & Spencer, Chocolate Muffin, in store bakery, each 23½
Pizza Hut, Triple Chocolate Muffin, each 19
Blackfriars Bakery, Monster Chocolate Muffin, 110g each 21½
McDonald's, Dessert, Double Chocolate Muffin, each 26
Waitrose, Triple Chocolate Muffin, 68g each 14
Costa Coffee , Mini Chocolate Muffin, 19g each 3½
Costa Coffee , Raspberry & White Chocolate Muffin, 136g each 25½
Costa Coffee , Triple Chocolate Muffin, 130g each 26½
J D Wetherspoon, Breakfast Chocolate Muffin, per serving 24½
Co-op, Double Chocolate Muffin, individual, in store bakery, 110g each 23
Greggs, Triple Chocolate Muffin, each 28½
Pizza Hut Restaurants, Triple Chocolate Muffin, each 19
ainsbury's, Chocolate Chip Muffins, 4 pack, in store bakery, 100g each 20½
Blackfriars Bakery, Chocolate Chip Muffin, 110g each 22
Tesco, Chocolate Chip Muffins, 4 pack, 72g each 14½