Brownie: where does the thin shiny crust come from

Useful to know

A brownie’s thin, shiny top is a separate feature from the texture underneath. King Arthur considers how ingredients and mixing influence that surface. [1]

Do not use the crust alone to decide whether the interior has the texture intended by the recipe. A fudgy brownie and a more cake-like one aim at different results. Compare the full method, quantities and doneness guidance rather than adding flour or extending baking solely to imitate a photograph.