Run the flour and sugar through a sieve on a flat surface.
3 cups All-Purpose Flour, ¼ cup Icing Sugar
Make a hole in the center (somewhat resembling a volcano) and add your salt. Cut the chilled, unsalted butter in small cubes, and throw them in the center, too.
¼ tsp Salt, 1 cup Butter
Start working the flour into the cold butter by squeezing it in with your fingers. You'll be squeezing cube by cube, so this process will take some time, but we need to get the butter mixed with the flour before the butter starts to melt.
Make another hole in the center of the mix and add in the egg yolks, walnuts and water. Start gently working everything together into a dough. You can use a kitchen machine for this step.
¼ cup Walnuts, 4 Egg Yolks, ½ cup Water
Once all those ingredients are nicely work together, cut the dough into two even balls. Wrap both of them in cling film and let them rest in a fridge for about an hour.
After the dough has chilled, put each ball between two sheets of baking paper.
Get out your rolling pin and roll them to about 3-4 mm / 0.1 inch thickness. You're going to use one for the base. Cut it with the cake pan ring, and place it on the cake pan base.
Use the second dough to make a longer oval shaped form with same thickness, and cut it about 4 cm / 1.5 inch wide (or according to your cake pan height). Use it to make the cake wall and press it on the bottom, so it connects with the base all around.
Apple Pastry Cream
Fill about half of a mixing bowl with water and your lemon juice, and let it mix it a bit.
1 Lemon
Peel the apples, and be sure to take out the pits!
2 Apples
Cut your apples into small cubes (about 0.5 cm / 0.2 inch). Drop the apple cubs into the bowl with the lemon water.
In a small pan, combine 50 grams / 1/4 cup of sugar and 5 tbsp. of the lemon water.
¾ cup Granulated Sugar
Heat up the sugar and lemon water on medium heat until the sugar fully dissolves.
Strain the apples from the rest of the lemon water mixture into a pan. You'll also be adding in the cinnamon and the roughly chopped walnuts in the pan.
1 tbsp Cinnamon, ½ cup Walnuts
Simmer them together on medium heat until the apples start to caramelize and begin to soften (about 5-10 minutes).
Let them cool down on the side.
In a separate medium bowl, whisk the egg yolks, whole eggs and sugar together, leaving enough space for the milk and heavy cream.
3 Egg Yolks, 3 Eggs, ¾ cup Granulated Sugar
Add in the corn starch to the mix and whisk well — you don't want to see any starch clumps!
¼ cup Corn Starch
Once that's been whisked well, you'll need to combine the milk, heavy cream and lemon zest (optional) in a larger pot. Be sure to leave some extra space in this case, as well!
1 ½ cup Whole Milk, ¾ cup Heavy Cream
Bring everything to a simmer, and then slowly pour it into your bowl with the egg and corn starch mix.
It's important that you're vigorously whisking while pouring!
Next, pour the mix back in the pot and cook on low to medium heat, all while constantly whisking until it starts to thicken. When the whisk starts to form ribbons, add in the vanilla extract and caramelized apples and walnuts. Take it off the flame.
1 tbsp Vanilla Extract
Make sure you don't over fill the cake pan, though, by leaving roughly 1 cm / 0.4 inch empty.
You'll need to bake your delicious custard apple tart at 180 °C / 356 °F for at least 40 minutes. For an indicator that your pastry is ready to come out of the oven, check if the surface of the custard starts to turn a golden brown. When you do take it out of the oven, the cake should be jiggly when shaken.
Let the cake to cool completely at room temperature before taking out the cake pan ring.
In order to really impress your guests, mix one tbs. of cinnamon with one tbsp. of icing sugar before sieving it over the cake!