There is an article in the May/June issue of Cook's Illustrated about this.
I'll write some of it here:
It took over a dozen calls and e-mails to food scientists, baking experts, and chocolate manufacturers to solve the mystery. Nestle had the best answer, which Dr. Alvin Huang elaborated on. During the first few minutes of baking, the fats in the batter melt and the sugar dissolves, giving batter increased fluidity. In this state, some of the sugar, rises to the surface. With baking it forms a crust in which the sugar molecules line up, results in smooth surface, and is shiny.
Corn syrup inhibits that, also too many eggs, also unsweetened chocolate.
Hope this helps some.