1-1/4 cup (188 grams) semi-sweet chocolate chips, divided
1 cup (200 grams) granulated sugar
3/4 cup (75 grams) cocoa powder
2 teaspoons (10 grams) instant espresso powder
3 eggs, room temperature
1/2 teaspoon (3 grams) vanilla extract
3/4 cup (90 grams) all-purpose flour
1/2 teaspoon (2 grams) kosher salt
1 cup (150 grams) chopped walnuts, divided
Heat oven to 350°F (176°C). Line a 23 centimeter square baking pan with parchment paper; set aside.
Melt together butter, 3/4 cup (125 grams) chocolate chips, sugar, cocoa powder and espresso powder in a large heat-proof bowl over a pot of simmering water, stirring occasionally until smooth. Remove from heat and set aside to cool for ten minutes.
Stir in eggs one at a time, just to combine. Stir in vanilla. Add flour, salt, 3/4 cup (112 grams) chopped walnuts and remaining chocolate chips. Pour batter into the prepared baking pan; spread evenly to edge of pan. Scatter with remaining walnuts.
Bake just until firm, 20 to 25 minutes. Cool completely before cutting to serve.