Preheat the oven to 350 F. Spray a standard-size loaf pan with a thin layer of oil and line with parchment paper.
Place the butter in a heatproof bowl and set in the microwave, heat it until it has just melted.
Add the sugar and whisk to combine. Return the bowl to the microwave for another 30 seconds. Whisk the mixture vigorously. It should be hot to touch, if not return it for another 10-20 seconds to the microwave.
Whisk in the cocoa, vanilla and salt. The mixture should still be warm but not really hot.
Add the eggs and whisk for a full minute, vigorously, until you have a shiny batter.
Fold in the chocolate and flour.
Pour into prepared pan and bake for 25-35 minutes.