In a large pot, fill with 2 inches of water and place a steam rack on the bottom. Heat on medium heat until it starts to simmer.

In a medium sized bowl, gently whisk together the eggs until thoroughly combined, trying your best not to introduce too many air bubbles.

In a separate bowl, combine dashi, soy sauce, mirin, and salt and stir to combine.

Slowly drizzle in the dashi mixture into the eggs, whisking gently until fully combined.

Strain this mixture through a sieve into a Pyrex measuring cup or any vessel with a spout to make pouring the mixture much easier.

Portion the egg mixture into individual cups (or your vessel of choice) until it is ⅔ full. Add in half of your filling of choice. Cover with lids (if available) or with aluminum foil.

Put your custard cups onto the steam rack. If your steam rack is not wide enough, you can place a plate on top of the steam rack and add your cups on top. Close the lid of your pot.

Steam for approximately 5 minutes and before the custard fully sets, open the lid and add the remainder of your ingredients/filling into the custard so that they peek through on top. Close the lids and continue steaming for another 5 minutes or until custard is fully set. You should be able to tap the surface of the custard with a spoon and it should remain somewhat firm when done.

You can serve this immediately while hot or at room temperature.