How to Make It
Heat oven to 350° F. Spray an 8-inch square baking pan with the nonstick spray. Line the pan with a piece of parchment, leaving an overhang on two sides; spray the parchment with the nonstick spray.
In a small saucepan, melt the butter and chopped chocolate with the espresso powder, stirring occasionally; remove from heat.
With an electric mixer, beat the eggs and sugar on medium-high until light and fluffy. Reduce speed to low and mix in the melted chocolate mixture, then the flour and salt just until incorporated. Fold in the walnuts and chocolate chips.
Spread the batter in the prepared pan and bake until a toothpick inserted in the center comes out clean, 30 minutes.
Cool completely in the pan. Holding both sides of the paper overhang, lift the cake out of the pan, transfer to a cutting board, and cut into 16 squares. Note: Total Time includes cooling time.