Place the butter and chocolate in a medium saucepan. Over low heat, stir until melted and smooth. Remove from heat and set aside to cool.
Add the vanilla, eggs, sugar and salt to the chocolate mixture. Mix until thoroughly combined.
Add the flour and mix just until combined and then stir in walnuts.
Spread batter evenly in prepared pan.
Bake for about 30 minutes. It may take longer, but start checking at 30 minutes. The top will be dry and toothpick inserted in center will come out almost clean.
Remove from the oven and cool on wire rack. Cut into 2" squares.