Preheat your oven to 160c (around 320f). Line an 8 inch square tin (okay, I used a circular one...) with butter and greaseproof paper.

Melt the butter in a bowl of water over a pan of just boiling water (don't let the bottom of the bowl touch the water). Once melted, add the sugar and cocoa powder and stir until you have a thick paste.

Add the sea salt (it helps, trust me!) and vanilla extract.

Allow the mixture to cool, then add the eggs one at a time, stirring vigorously with a wooden spoon. The mixture should now be a glossy mix (like the first picture). If you add them before the mixture is cooled enough, the eggs will scramble, so be careful!

Fold in the flour, then mix vigorously again. Pour into your lined tin, then place in the preheated oven for around 35 minutes or until a skewer comes out clean.