Preheat oven to 335.
Line an 8x8’ square pan with parchment paper.
Process the chocolate in a food processor until its partially powdered, partially chopped (this should take less than a minute).
Over a double boiler add the butter. Once it has fully melted, add the cocoa and sugar.
Stir until combined then cook until the mixture registers 140 F (some of the liquid will begin to separate). This will take less than 5 minutes.
Remove from heat and pour into a bowl. Let it cool until it is just warm to touch.
Stir in the salt and vanilla.
Add the eggs and beat with a hand mixer until the mixture is smooth and silky.
Add the flour and processed chocolate and stir it with a hand-held whisk.
Pour into baking pan and bake for 40-45 minutes.
Allow to cool completely in the pan and then use the sling to lift it out.
Slice into 1 inch squares.