In a medium heatproof bowl placed over a pot of simmering water, melt both the bittersweet and unsweetened chocolates until melted. Allow the mixture to cool for about 10 minutes.
Whisk the granulated sugar, the eggs, and the vanilla into the melted chocolate. Mix well.
Add the all-purpose flour and salt.
Fold in the semisweet chocolate chips.
Pour batter into a greased 8-inch square baking pan lined with foil and greased again.
Bake for 25-28 minutes or until a small knife inserted into the center comes out with only a few crumbs attached. Do not over bake. Cool.
Chill the pan of brownies in the refrigerator for a few hours. Lift the foil out of the pan and peel it off. Cut brownies into squares.