Pulse butter dough ingredients in a food processor until well incorporated. Remove and wrap the mixture in plastic wrap. It will be crumbly. While the mixture is in the plastic wrap, mash the dough to form a rectangular shape as best you can. Refrigerate your dough for 20 minutes.
Next, add all your water dough ingredients to the food processor and pulse until all ingredients are incorporated. Remove dough from food processor and roll out the dough into a circular shape. It doesn’t have to be perfect. Take the butter dough out of the fridge and place it in the center on top of your water dough. Fold the water dough over the butter dough on all four sides until the butter dough is completed wrapped up in the water dough.
With your rolling pin, roll the now combined doughs flat to about 1/2-in thickness. Fold the dough into thirds. Wrap it in plastic wrap and refrigerate for another 20 minutes.The dough can still be crumbly, and that’s OK.
Roll out dough again into a rectangle, 1/4-in thick. This time fold the dough using a gate-fold and then fold it again in half so you can see four layers on one side. Refrigerate for 15 minutes.
Repeat step 4 again, rolling out the dough, 1/4-in thick.
In a small bowl, mix the sugar and water together and microwave for about a minute. Set the bowl aside to cool down to room temperature.
Whisk the custard filling ingredients together and then mix in the water/sugar mixture.
Drain the mixture using a sieve. This will ensure a smooth custard filling and prevent any bubbles from forming.
Preheat your oven to 400 degrees Fahrenheit.
Take the dough out of the fridge and roll it out to 1/4-in thickness. Using a 3-in cookie cutter (or one slightly larger than your mold), cut out circles.
Lay cut-out dough circles into each mold, pressing the dough gently against the mold.
Take a fork and prick the bottom of each mold. This is to keep the tart from puffing or rising too much.
Fill about 80% of each tart with your egg mixture.
Place the molds on a baking pan and bake the tarts for 10 minutes at 400 degrees Fahrenheit. Turn down the heat to 350 degrees and bake for another 15 minutes. At this point you will want to watch the pastry carefully to prevent the custard from burning or puffing up too much. You can also crack open the oven door for the last five minutes to help prevent the custard from burning.
Cool for 5 minutes before serving.
Skip the steps for the traditional filling and instead do the following: Whisk together 4 eggs, 2 egg whites, 2/3 cup of evaporated milk, 1/2 tsp vanilla extract, 2 T orange zest, 1 tsp. grated fresh ginger, 1/2 cup sugar, 1/2 cup freshly squeezed juice from half an orange. Drain the mixture using a sieve and set aside until you are ready to assemble tarts.