Place a baking sheet (parchment paper) in a 9x12 inch pan so it covers the bottom and 2 sides (to facilitate the brownie removal). Butter the pan and set aside.
Measure 1/4 of hot water, add to it 1 teaspoon instant coffee. Set aside.
In a mixer fitted with the wire whisk attachment, beat the eggs at medium speed until fluffy and light yellow, add the white and brown sugar and beat until combined. Add remaining ingredients except chopped walnuts, and mix to combine.
With a spatula, stir in the walnuts.
Pour the batter into the pan and bake for 35-40 minutes. Check for doneness by inserting a toothpick into the center of the pan, it should come out with just a few crumbles attached. I find that 40 minutes in my oven for this recipe is perfect timing.
When done, remove from oven and place the pan on a cooling rack.