In a small bowl, combine the custard powder and a few tablespoons of soy milk until there are no lumps.
Add all the custard ingredients to a medium-size saucepan over medium-high heat. Whisk consistently while the custard heats up. Gently boil the custard for about 3-5 minutes then remove from the heat.
Your custard may have little clumps or bubbles throughout. If so, place a sieve over a wide pouring jug and carefully pour the hot custard in the sieve. The leftover custard should be smooth and free from bubbles.
Place all your baked tart shells (while they're still in their tins) on a large tray or in a container. Gently pour the custard into each tart shell. Allow them to sit at room temperature for 5 minutes then transfer them to the fridge for 1-2 hours or until set. The tarts will also set at room temperature but the fridge speeds up the process.
The tarts are set if you can insert a toothpick in the middle and the toothpick stands by itself. The tarts are ready to serve! The tarts can be kept at room temperature for a few hours before serving. Alternatively, store the egg tarts in the fridge in an airtight container for up to 5 days.