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 or in a pot set over low 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 mix to just combine, careful not to overmix.
Pour into baking pan and bake until the center has puffed up and/or a cake tester inserted into the center comes out with some wet crumbs, but is not overly gooey, about 40-45 minutes depending on the size of your pan(less if you're using a 9x9, more if using an 8x8).
Allow to cool in the pan and then use the parchment sling to lift it out.
Slice into 1 inch squares.