IMPORTANT TIPS

1. Preheat the oven to 180 degrees Celsius (356 degrees Fahrenheit).
2. Using a non-stick spray, spray or coat a deep wide baking pan (as I mentioned in the tips section, you need space over the cake while still in the baking pan, to add a layer of the milk tart filling on top).
3. Continue by combining the all-purpose flour, salt, and baking powder in a large bowl.
4. Separate the eggs into 2 other bowls.
5. Beat the egg yolks with 3/4 cup of castor sugar on high speed, until the yolks are pale yellow.
6. Properly stir in the milk and vanilla essence into the egg yolk mixture.
7. Pour the egg yolk mixture onto the flour mixture and gently fold until properly combined.
8. Next, beat the egg whites at a high speed until soft peaks start to form.
9. While the mixer is turned on, add the remaining 1/4 cup castor sugar to the egg whites and beat until the egg whites are stiff (be careful not to overbeat your egg whites or they'll go dry).
10. Gently fold the egg white mixture into the batter until it's properly combined.
11. Pour the batter into the prepared pan and spread it evenly over the surface.
12. Pop it in the oven and bake for 25–30 minutes or till your toothpick comes out clean when you check it (keep checking once you hit the 20-minute mark).
13. While the cake is baking, combine in a pitcher your evaporated milk, fresh cream, and sweetened condensed milk, and keep aside. 
14. Now it's time to make the milk tart filling. In a deep medium-sized pot, add the milk, butter, sweetended condensed milk, and powdered cinnamon. Heat up the mixture on medium heat till it's warm—do not let it start boiling. 
15. While the mixture is heating up, in a bowl, add your eggs, cornflour, and vanilla. Beat the mixture till it all comes together. 
16. Once your milk mixture has heated up, take it off the stove and slowly pour the egg mixture in a continuous manner into the pot whilst at the same time whisking vigorously. This will keep the eggs from scrambling as they combine with the hot milk mixture. 
17. Next, on medium heat, place the pot back on the stove and whisk continuously until the mixture has thickened. 
18. Remove the from the heat, once the milk tart filling has thickened and leave it to cool down. 
19. When the cake has finished baking, also leave it to cool down before poking several holes on the surface with a toothpick or fork and slowly drizzling 1 1/2 cups of the milk mixture (that you made in a pitcher) all over the cake and don't forget the edges. 
20. Time to make the whipped cream. Start to whip the fresh cream and you'll see it begin to thicken up. As it comes closer to stiffening, slowly add in the icing sugar and continue whipping. You want the whipped fresh cream to taste sweet and also maintain stiff peaks. Be careful not to overwhip your fresh cream because it may split. 
21. When your milk tart filling has cooled down (it's okay if it's a bit warm), give it a quick mix and pour the filling over your cake, spreading it out evenly. 
22. Pop the cake in the fridge for 1–2 hours just so the milk tart filling can start to set. 
23. Next, pipe a thin layer of whipped cream over the milk tart filling. You can see from my picture that the whipped cream was quite a lot. After tasting, I feel a thinner layer of whipped cream would complement the cake along with the milk tart layer much better. I already adjusted the ingredient list to half the amount I initially used - this is just a quick note in case you wonder why your whipped cream isn't as much as in my pictures. 
24. Gently sift some powdered cinnamon over the whipped cream. I went on to also decorate with some edible rose gold leaf. 
25. Place the cake back in the fridge. I'd suggest overnight so the milk tart filling sets properly. 
26. Finally, when serving the milk tart tres leches cake don't forget to serve it with the leftover milk mixture that was in the pitcher, so everyone can pour some over their cake slice, and enjoy.