To make the dough shortbread, place the icing sugar and butter in a food mixer and mix together using the paddle attachment. Add the fresh thyme and lemon zest to the sugar and butter along with the flour, egg yolk and salt. Mix together until a ball of dough is formed.
Turn out the dough onto a piece of cling film. Shape into a thick disk then wrap in the cling film and place into the fridge for at least an hour.
TIP:The dough may also be made the day before. If making the dough the night before remove the dough from the fridge 30 mins before rolling to allow the dough to soften.
Preheat the oven to 195°C / 175°C fan assisted and line two baking trays with parchment paper.
Roll out the dough on a well-floured surface, moving the dough each time you roll it to make sure it’s not sticking, to 5mm thick. Cut using a round or square cutter depending on what you have available, the size of your custard creams is up to you.
Transfer the cut dough onto the prepared baking trays and bake for 12 - 14 minutes, until lightly golden.
Once baked, transfer the biscuits to a wire rack to cool completely.
For the crème patisserie, combine milk, cream and vanilla in a large saucepan. Bring to a simmer over medium heat.
While the milk and cream mixture is heating, whisk together icing sugar, flour, whole eggs and egg yolks in a large bowl to form a smooth paste. You don’t need to whisk to a ribbon stage as we are not incorporating any air in the mixture.
Once liquid has come to a simmer, remove from heat. Pour about half of the hot liquid into the egg mixture and whisk to combine. Pour the egg mixture back into the saucepan with the remaining liquid and whisk well to combine.
Return to medium heat and cook, constantly stirring with a whisk, until very thick. Tip: Even when you think the mixture is thick enough, keep over heat and whisk for another minute to ensure it is thick enough.
Transfer hot crème patisserie to a heat resistant bowl and cover with cling film, making sure the cling film is pressed against the surface of the custard to make sure a skin doesn’t form.
Refrigerate for 30 minutes to cool down.
Remove from the fridge and peel away cling film, it should still be slightly warm but not hot.
Add the butter and use a stick blender to incorporate it into the custard. Blend until completely smooth. Cover with clingfilm, again, pressed against the surface, and chill in the fridge until needed.
To assemble the custard creams, spoon crème patisserie into a large piping bag with a plain round piping tip, or simply just trim the end off if using a disposable bag. Pipe crème patisserie directly into the centre of half the biscuits (which should be turned over) then sandwich the biscuits with another shortbread on top, pressing gently so that the custard spreads to the edges of the custard creams.