Preheat the oven to 350°F (180°C.) Grease a 9-inch (23 cm) square baking dish with butter. Line bottom and sides with parchment paper, leaving overhang; set aside.
Place chopped unsweetened chocolate, butter and salt in heatproof bowl set over barely simmer water. Heat, stirring occasionally, until melted and smooth. Remove from heat; stir in granulated and brown sugar. Cool slightly and set aside.
Stir each egg into the chocolate mixture, one at a time, until well combined; stir in vanilla. Stir in flour until combined. Fold in walnuts and chopped semisweet chocolate. Scrape the batter into the prepared dish; smooth top.
Bake in the centre of the oven for 25 to 30 min or until only a few moist crumbs stick to a toothpick when inserted into the centre. Cool completely in baking dish.
Using parchment paper as handles, remove brownies from dish. Peel off parchment paper. Cut into 16 squares.
Store, refrigerated, in airtight container for up to 3 days or freeze for up to 6 months.