Crack the eggs into a medium bowl, and whisk together lightly.

Using the same saucepan as before, melt the butter and chocolate together.

Whilst this mixture is still warm, pour about 4 tbsp of it, 1 tbsp at a time, into the eggs, whisking the eggs constantly. This tempers the eggs :)

Add the tempered egg mixture to the butter-chocolate mixture in the saucepan and whisk together.

Whisk in the salt and powdered sugar, then return the pan to a medium-low heat and stir constantly with a spatula (making sure to scrape the sides + base of pan) for 5 minutes (this kills any bacteria by partially cooking the batter).

Stir in the vanilla extract, flour and milk and whisk until smooth.

Pour over the still-warm cake and spread it out evenly, sprinkle on the chocolate chips (if using) and leave to cool to room temperature. To store: cover the pan with plastic wrap and refrigerate.