In a pot, pour in water, add in salt, and butter, bring to boil
Add in flour, a little at a time, until it become a dough, then take off the pot from heat
Let cool for 8-10 minutes
With a spiral head of an electric mixer, cream in eggs, 1 at a time, until completely combined, then spoon dough into piping bag
Grease baking pan and pipe dough as big as you like, then bake for 20 minutes on preheated 390F
Meanwhile, make the custard following the instructions on the packaging (6 mins on microwave in total)
Cool puff on cooling rack, and once cooled, fill the center with custard