See Lemon Curd.
Soften the gelatin in the water and set aside.
Heat the milk until hot but not boiling.
Whisk the egg yolks and sugar together in a non-reactive saucepan.
Whisk in the flour.
Add the milk slowly, whisking well and scraping the corners of pan with a rubber spatula to make sure all of the egg yolk mixture is incorporated.
Place over medium heat and, stirring constantly, bring to a boil. Boil for 1 minute, stirring constantly.
Remove from the heat and add the vanilla (if using extract). Tear the gelatin into small pieces and stir it into the hot liquid (there is no need to liquefy it, the heat of the mixture will do this for you).
Pour into a storage container,
cover the surface with plastic wrap, poke a few holes in it and refrigerate overnight to chill or freeze.
Dissolve the gelatin in the water.
Place the eggs and yolks in the top of a double boiler.
Whisk to combine.
Whisk in the sugar and lemon juice.
Add the butter pieces.
Place over hot water and, stirring constantly,
bring to 172 degrees on a candy thermometer.
Tear the gelatin into little pieces and whisk into the curd.
Use at once or pour into a storage container, cover the surface with plastic wrap;
poke a few holes in it and refrigerate to cool.