The custard = as gingerbread has flour in it, if it is not strained well you will get a thick mixture, i feel it is better to use a nut/straining bag to make this. I have made it with out one and the cookie/biscuits disintegrated into a thick sludge. I had to strain it about 5 times and it was still very thick, it tasted nice though and didn't require any additional flour. If you find after staining that you have a thick mixture then reduce the amount of corn flour.
Its important to use diced butter for the choux pastry because you want it to melt before the water boils so it doesn't evaporate.
Craquelin - roll craquelin as thinly as possible between two sheets of baking paper. If too thick it can weight it down. you can re roll it and cut more if needed.
Eggs = the quantity of eggs can vary with choux pastry. If you release too much steam, or your flour has moisture in, a number of things can affect it so you need to use your insight and add what you feel is the right amount. You are looking for a smooth shiny dough that forms a v shape when you hold spoon up. It should hang there for a moment then fall off.
steam helps them rise so sprinkle a few drops of water on the tray.
You must poke holes in the buns and put them back in the oven to help dry the insides out