Line an 9×9 inch square pan with foil,lightly butter and set aside.
In a small saucepan set over low heat, melt butter, then add the sugar.
Once the sugar is stirred in, add the chopped chocolate, stirring until smooth.
Remove from the heat and add the eggs, one at a time stirring well, then the vanilla extract.
Sift the cocoa powder and cornstarch together in another bowl.
Add to the saucepan, add the salt.
Stir together, then beat vigorously for 1-2 minutes.
Pour the batter into the prepared pan and bake for 20-25 minutes. (be careful not to over-bake). Remove from oven and let cool in the pan for 45 minutes before removing and slicing.