Set the sugar in a medium sized pot. Add the egg yolks and immediately whisk very very well (or use an immersion blender) until light and creamy.

Add the heavy cream and milk and whisk to combine.

Sift in the cocoa and add the salt then whisk until incorporated - it takes a few minutes to get the cocoa well mixed.

Set the pot over medium heat and cook slowly and evenly, until the custard reaches 170 F (use a thermometer, instant read or laser to check). It will cook quicker on the bottom so stir often so it does not curdle or burn.

Turn off the heat and add the chunks of chocolate and the vanilla. Use either a hand held immersion blender or transfer the mix to a blender to blend until smooth.

Pour the mix through a fine mesh sieve into a heat proof bowl. This is to remove any lingering bits. Let it come to room temperature then cover and chill for 8 or up to 48 hours.

The custard will have settled unevenly so give it a good whisk before pouring it into your ice cream machine. Follow machine instructions to churn. Ice cream should be stored in an airtight container in the freezer.