Preheat oven to 350°F with a rack in the center. Use cooking spray or butter to lightly grease an 8-inch x 8-inch baking pan, then lightly flour it.
Place the chocolate chips and butter into a large microwave-safe bowl. Heat on half power until just melted — chips may retain their shape until stirred. This takes about 2 minutes in my microwave, but your mileage may vary. Stir until completely smooth. Stir in the brown sugar, coffee and vanilla until smooth. Crack the eggs into the bowl and mix very well into the batter.
Sprinkle the flour, cocoa powder, baking powder and salt evenly over the chocolate mixture, then stir until well combined and shiny. Stir in nuts if using. Scrape batter into prepared pan.
Bake for about 30 minutes, until a tester comes out with a few crumbs but not a lot of molten chocolate. Let cool in the pan before cutting into 16 squares to serve.