Step 1Toast nuts in dry skillet set over medium heat until lightly browned, 3 to 4 minutes. Set aside.
Step 2Microwave chocolate until melted, about 1 minute. Set aside until cooled to room temperature.
Step 3Beat butter and sugar on medium speed until very well mixed. Add 1 egg and mix until almost incorporated. Add remaining egg and mix until just combined. Scrape sides of bowl. Add vanilla and melted chocolate and mix 30 seconds.
Step 4Mix in flour and salt on low speed until almost combined, with some flour still visible. Using spatula, fold in chocolate chips and nuts. Pour into greased and floured 9-inch pan. Smooth top.
Step 5Bake at 350 degrees until center stops rising and toothpick comes out with a few moist crumbs, about 27 minutes. Cool to room temperature. Cut into 12 squares, or refrigerate overnight, covered airtight up to 3 days. (Can be frozen up to 2 weeks.)