Vegetable-oil cooking spray
¾ cup chopped walnuts
1 cup cake flour or ¾ cup plain flour
¼ cup unsweetened cocoa powder
¼ teaspoon salt
1 teaspoon baking powder
1 tablespoons finely ground espresso beans
3 ounces unsweetened chocolate, finely chopped
¾ cup unsalted butter at room temperature
1½ cups sugar
3 large eggs, lightly beaten
1 teaspoon vanilla extract
Spray an 8-by-8-inch baking dish with cooking spray. Line the pan with aluminum foil, then spray the foil with cooking spray.
Spread the walnuts on a rimmed baking sheet and toast in the oven at 140C until lightly browned, about fifteen minutes. Set aside to cool.
Fill a medium saucepan one-third full with water and bring to a simmer.
Place the chocolate and butter in a large heat-proof bowl and set over the simmering water. Turn the heat to low. Melt the chocolate and butter, stirring frequently, until completely smooth. Remove from heat and stir in the sugar.
Whisk in the eggs and vanilla.
In a mixing bowl, sift together the flour, cocoa powder, coffee ground, salt and baking powder.
Add the flour mixture and the walnuts to the chocolate mixture and whisk until well combined.
Pour the batter evenly into the pan. Bake at 160C or until a crust forms on top and the center is still slightly gooey, about 15- 20 minutes.
Cool in the pan set on a rack.
Refrigerate two hours.
Cut into 1-inch squares to serve.