Preheat oven to 180 ° C/375˚F/gas mark 4.
Firstly prepare the craquelin. Place the softened butter into a bowl and add the sugar and flour, combine to create a dough. Then roll out the dough thinly (1 mm thick) between 2 sheets of parchment paper. Keep in the freezer until required.
In a saucepan, combine the milk, sugar, and the diced. Bring the mixture to a boil, then remove from heat.
Add flour all at once and stir quickly with a spatula or wooden spoon until all the flour is fully incorporated. Remove the pan from the heat and pour the contents into a bowl, then add the eggs one by one mixing vigorously.
Spoon the choux pastry into a piping bag and pipe walnut size pieces on a baking sheet lined with parchment paper.
Cut out discs of craquelin to the size of the choux and place onto the choux.
Bake the choux for 20 minutes and leave to cool.
When the choux are cooled, prepare the fondant mixture by melting the fondant with the colorant in a bowl over a pan of simmering water. Once softened and combined, dip the top of the choux and set aside to cool.
Finally, combine the lemon curd and crème patissière in a bowl and fill the choux either using a piping bag fitted with a nozzle or by slicing the choux by half.
