Use a cookie cutter or any cups that have about 3.5-inch diameter to cut out about 18 circles from your puff pastry sheets. Mold this over aluminum muffin cups. Place these back in the freezer
Place all the ingredients for egg custard filling in a mixing bowl. Use a balloon whisk to whisk until smooth. Prepare a water bath that you can put your saucepan in later. Transfer this to a medium saucepan over low heat and use a rubber spatula to keep stirring until the mixture started to coat the spatula thinly and the sides of the pan. Remove from the heat and transfer the saucepan to a water bath to let it cool down quickly
Make sure your oven is clean and no bits or anything that might burn in there and set off your smoke alarm (yikes!). Preheat your oven to 485 F (yes, it's pretty high temperature). While the egg custard mixture is ready, remove the puff pastry from the freezer and place this on top of a baking sheet lined with aluminum foil. pour the egg custard mixture until about 3/4 full in each shell
Place the egg custard in the oven, 3rd rack from the top and let it bake for 30 minutes. You want to rotate it every 10 minutes for even browning. Towards the end of cooking, the custard will start to bubble and blistered from the high heat
Remove from the oven and very carefully. Let it cool down a bit for about 1 minute and then unmould the tarts to a wire rack to let them cool down. They will turn soggy if they sit in the muffin cups for too long. The top will sink a little bit and this is normal, don't be alarm. Serve while warm if possible. The outside will be flaky and the filling is creamy and smooth
Notes
You can store the completely cool down tarts in the refrigerator and reheat them in the oven at 400 F for about 10 minutes. I have tried using air fryer to reheat too, I know it sounds weird because we are not frying anything but it works too ;)