Craquelin is a crispy cookie dough topping that's rolled thin and placed on choux pastry before baking. Josh used this advanced technique, creating a mixture of butter, brown sugar, flour, and cocoa powder that's rolled to just a couple millimeters thickness. This creates an incredibly crispy top layer that contrasts beautifully with the soft pastry underneath. The craquelin prevents the choux from expanding unevenly and gives a professional bakery appearance with superior texture and visual appeal.