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 line it with a piece of parchment that extends enough on two opposite sides to form handles. (You'll use these later to lift the brownies out of the pan.)
Place one generous cup (7 ounces) of the chocolate chips and the 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. Alternatively, you can set the bowl over a pot filled with an inch of simmering water.
Stir until completely smooth.
Add the sugar and brown sugar and beat with an electric mixer for one full minute.
Crack the eggs into the bowl, add the coffee, vanilla, and salt, and beat for another full minute.
Sift the flour, cocoa powder, and baking powder into the bowl. Using a spatula or wooden spoon, stir gently until incorporated.
Stir in remaining chocolate chips, along with nuts, if using.
Scrape batter into prepared pan.
Bake for 30 minutes. Batter will be set on top and around the sides, but a tester will not come out clean.
Let cool in the pan set over a wire rack for at least 30 minutes.
Run a knife carefully around the edges of the pan, and use the parchment handles to lift the brownies out of the pan and onto a cutting board.
Cut into 16 squares to serve.