Cooking Out & Hydrating Choux (Cook the Panade, Pipe to a V-Drop)

Flat choux almost always comes down to too much water left in the paste, or too much egg beaten in. After adding the flour off the heat, return the pan to medium heat and cook the paste for two to three minutes, stirring constantly, until it films the base of the pan and pulls cleanly from the spoon — this dries the panade so the buns can hold their rise. 

Then judge the egg by feel, not by count: beat it in gradually and stop when the paste falls in a smooth ribbon that leaves a slowly closing V. 

A hot first phase in the oven drives the rise, and keeping the door shut through that first set stops it collapsing back down.