Preheat the oven to 350 degrees F (175 degrees C). Spray an 8x8-inch baking dish with cooking spray. Line the pan with aluminum foil; spray foil with cooking spray.

Place walnuts on a baking sheet.

Toast walnuts in the preheated oven until lightly browned, about 6 minutes. Remove from the oven and set aside to cool. Leave oven on.

Sift flour and cocoa powder together in a bowl. Stir in coffee. Set aside.

Fill a saucepan 1/3 of the way full with water and bring to a simmer over medium-low heat. Place butter and chocolate in a large heat-proof bowl and set the bowl over the simmering water. Reduce heat to low and stir chocolate mixture frequently until completely smooth, 3 to 4 minutes.

Remove bowl from heat and stir in sugar. Whisk in eggs and vanilla extract. Stir in flour mixture just until absorbed; stir in toasted walnuts. Pour batter evenly into the prepared baking dish.

Bake in the preheated oven until a crust forms on top and center is still slightly gooey, 30 to 35 minutes.

Remove from the oven and set the pan on a wire rack to cool to completely, about 30 minutes. Refrigerate 2 hours to overnight; cut into squares.