Set up your steamer and get the water in the base boiling.
To make the dashi, bring 600 ml water to the boil.
Whisk in the instant dashi powder, miso paste if using, soy and mirin and simmer gently for 30 seconds, then turn off and allow to cool.
Lightly beat the eggs in a bowl then pour in the cooled stock and lightly mix with chopsticks.
Strain through a fine sieve and divide between four cups or bowls (pop any air bubbles with the tip of a knife).
Place in the steamer, cover with a sheet of baking paper and reduce the heat immediately, allowing the custards to steam lightly for 8 to 10 minutes or until they tremble slightly when moved.
Carefully remove, place on a serving plate, and garnish with the cooked broad beans and prawns.
Pod the broad beans and cook in a pot of simmering salted water for 2 minutes, then scoop them out and cool in cold water.
Peel off the outer layer, revealing the bright green inner beans.
Drop the peeled prawns into the simmering salted water and poach for 1 minute until just pink, then remove; they will keep cooking as they cool.
Toss the prawns and broad beans in a little sesame oil, and arrange on top of the custards.
