Both the biscuit and the chocolate have to be blessed and one need pronounce only the blessing over the main components, thus for a chocolate croissant one would say the blessing over the dough, and skip the blessing over the chocolate.
Blessing both the biscuit and the chocolate is essential; one should pronounce the blessing only over the main components. Therefore, for a chocolate croissant, you would bless the dough and omit the blessing over the chocolate.