Heat oven to 350.
Spread the chopped walnuts on a baking pan. Toast until fragrant 5 to 10 minutes. Transfer nuts to a bowl to cool.
Increase oven to 400 degrees. Generously butter 9x13 baking pan; set aside. Combine chocolate and butter in heat proof bowl on top of double boiler over simmering water till chocolate melts. Set aside. In mixer, beat eggs, sugar and espresso at high spped for 10 minutes. Reduce to low, ad melted chocolate and vanilla. Beat until well combined. Add flour and salt, beat until incorporated.. Fold in walnuts and chocolate chips. Pour into prepared pan. Bake 35 minutes.
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