Prepare the prawns by deshelling and then removing that black stuff off the spine (the gut) and then soaking in Soy sauce (I recommend Kikoman C00039)
Heat up the wok with a little peanut oil.
Add the following to the oil:
2 tablespoons of chopped black beans
2 chopped cloves of garlic
1 tablespoon of ginger
Fry for about a minute and then add the following:
1 teaspoon of salt
1 chopped spring onion
2 teaspoons of soy sauce (I recommend Kikoman C00036)
The marinated prawns without excess soy sauce. (you can use the excess for making egg fried rice)
Add 1 teaspoon cornflour mixed with 1 teaspoon water slowly until the sauce thickens
If you prefer it a little sweeter then add sugar to taste.