Lightly grease a 12-hole muffin tin with oil spray or melted butter (mine is a regular sized muffin tin, each hole has 1/3 cup capacity).
Get your pastry out of the freezer and put it on the bench to defrost while you make the custard.
Whisk together the eggs, sugar and cornflour in a saucepan (off the heat) until smooth. Gradually whisk in the milk, cream and vanilla – if you dump it all in at once you’ll have lumpy custard. No-one wants that.
Place the saucepan over low heat and cook, stirring, until it thickens and comes almost to a simmer (don't let it boil or the eggs will curdle). It doesn’t need to be really thick as it’ll cook more in the oven. If it coats the back of a spoon without running straight off, you’re probably good.
Pour the custard into a bowl and cover directly over the top with cling film. Make sure the film is touching the top of the custard as this will stop a skin forming. Place in the fridge for about an hour to cool.
Now for the pastry. Sprinkle about a tablespoon of extra sugar and half of the cinnamon over one sheet of pastry (you could combine them first but I just roughly scattered each of them individually). Put the other piece of pastry on top and gently squash them together with a rolling pin. Sprinkle another tablespoon of sugar and the remaining cinnamon over the top, then roll the pastry tightly into a log and put in the fridge to chill until you’re ready to cook the tarts (or until the custard is cool). You can skip the chilling here, but it makes the next step easier, and if you’re cooling the custard anyway then why not.
When you’re ready to cook, set your oven to 400⁰F/200⁰C, combi steam. If your oven has variable steam settings, use 50-60% steam. If not, don't worry! Just set to combi steam at the correct temperature and the oven will figure out the humidity for you.
Cut the pastry into 12 even scrolls. Place one, cut side up on the bench and use the heel of your hand to flatten it into a disc which will fit your muffin tin – don’t worry about all your swirls being evenly squashed, the unevenness is part of the appeal.
Repeat with the other pieces of pastry, then put them into the prepared muffin pan and pour the custard evenly between the pastry cases. Bake for 20-22 minutes, until the pastry is puffed and golden, and the custard is lightly browned on top. During cooking the tarts will puff quite dramatically, then they'll sink and settle after you remove them from the oven.
Transfer the tarts to a wire rack to cool for a few minutes, then turn out and eat warm or at room temperature.