Start by placing the flour and salt into a large bowl and whisk gently to combine. Add the frozen butter and cut into the flour using a pastry blender or two knives. You want the butter to be in small pieces, but still solid. Make sure all of the butter pieces are coated in flour.

Once the butter is cut into the flour, sprinkle the cold water evenly over the mixture. Using a knife or your hands, gently work the mixture until all the flour is moistened and it can be pressed together and hold its shape. Be gentle with it from this point on, because the more you work it, the more the gluten strands in the flour develop and the tougher your pastry will be.

Turn the dough out onto a floured surface. With a floured rolling pin, roll the dough into a square. Wrap the square and let it rest in the fridge for about 15 minutes to allow the butter to firm up further. Then, roll your square into a thin rectangle. With the short end of the dough facing you, fold the rectangle like you would a letter (bottom third up over the middle, then top third down over the other two thirds). Rewrap the dough and place back in the fridge for 20 minutes. Roll the dough out into another rectangle, and complete the letter fold again. Chill again, then repeat this process one more time.

Once you have folded and rolled the dough three times, roll it out into a rectangle again. With the long edge facing you this time, roll the dough tightly into a log, rewrap it, and place it in the fridge to chill for at least an hour so that the butter is firm and ready for what's to come!

In a medium saucepan, combine the egg yolks, milk, cream, sugar, and flour, and whisk to combine. Add the cinnamon stick and salt, then put the saucepan over a low heat. Whisk gently and continuously as the custard heats to prevent overcooking. You want the egg yolks to thicken the custard, not cook into scrambled eggs!

Keep whisking until the custard reaches about 150˚F. If you don't have a thermometer, you want it to thicken to the stage of being like a runny pudding. We don't want to cook and set it outright, as it still has to go into the oven! You can start out at a low heat, but gradually raise it over time if your custard just won't thicken. It should take about 15 minutes, but when I passed that mark without a change in texture, I raised the temperature gradually to a medium-low to get it to the needed 150˚. When it's at the runny pudding stage, take the custard off the heat and whisk a bit to help it cool. As it cools, stir it every so often to prevent a skin from forming on the top. Remove the cinnamon stick once cooled.

Preheat your oven to 550˚F and position a rack in the middle of the oven. Lightly spray a 12-cup muffin tin with nonstick cooking spray and set aside.

Remove your dough log from the fridge and trim the edges so it's nice and neat. Make 12 evenly-spaced marks on the dough and then divide the log along those lines to give you 12 pieces, ideally with a nice swirl pattern from being rolled into a log.

Being careful to handle the dough as little as possible to prevent the butter from warming, make each dough piece into a bowl by pushing down with your fingers. Thin out the sides, twisting the bowl as you go, until your pastry is as thin as you can get it without breaking it. Place each bowl into the muffin tin slots. They should ideally come to just above the rim of each slot, but if yours are a little smaller, that's ok! Keep in mind the smaller the tart, the quicker it will cook.

Carefully pour the cooled custard into each cup, filling it about 3/4 of the way full. Wipe away any drippings you get on the pan, because you don't want to wash off burnt-on custard! Place the muffin tin into the oven and bake for about 12 minutes, until the pastry is browned and the custard has a slight wobble to it.

At this point, you'll want to achieve some light browning on the top. This can be difficult to do at home without a blow torch, but if you keep the rack at the middle position in your oven, you should be able to manage it without burning the whole tart. Turn your broiler on high if your oven has that feature, and carefully watch the tops of the tarts. You want a few brown spots, but nothing overly burnt. When you start to see some browning on the custard itself, turn the broiler off and remove the tarts from the oven.

Allow the tarts to cool for about 10 minutes, then gently pop them from the tin using a knife to loosen the edges and let them cool on a wire rack. The custard will firm up the longer they sit, but these are best eaten slightly warm and fresh, so the happy medium point is about 15 minutes of cooling. My favorite way to serve these is with a light dusting of cinnamon on the top. Enjoy a little trip to Portugal with every bite!