Preheat oven to 335.
Line a 9×9″ or 8×8’ square pan with a parchment paper overhang (two overlaying sheets so the pan is completely covered).
Over a double boiler, on medium heat, melt the butter. When it has fully melted, add the the sugar and cocoa.
Stir and continue to cook until the mixture is smooth and registers 140 F. You’ll notice that at this temperature the liquid starts to separate a bit.
Remove from heat and pour into a bowl. Let it cool slightly, until it is warm to touch, so it doesn’t cook the eggs in the next steps.
Stir in the salt and vanilla.
Add the eggs and beat with a hand mixer until the mixture is just smooth and silky (about 20 seconds). You can also do this with a whisk.
Add the flour and whisk it in by hand.
Pour into baking pan and bake for 40-50 minutes.
Allow to cool fully in the pan and then use the parchment sling to lift it out.
Slice into 1 inch squares.