Position a rack in the middle of the oven and preheat the oven to 350°F. Line a 9-by-13-by-2-inch baking pan with parchment, allowing the ends of the paper to hang over two opposite edges of the pan.
Whisk the flour and baking powder together in a small bowl and set aside.
Put 1 cup of the chocolate chips, the unsweetened chocolate, and the butter in a large heatproof bowl, set it over a pot of barely simmering water (do not let the bottom of the bowl touch the water), and stir frequently until the chocolate and butter are melted and smooth. Remove from the heat.
Meanwhile, in a medium bowl, whisk together the eggs, espresso powder, vanilla, and sugar until thoroughly combined.
Add the egg mixture to the chocolate mixture and stir with a wooden spoon until the mixture thickens, 3 to 5 minutes. Sprinkle the flour mixture over the chocolate and mix it until combined. Stir in the walnuts and the remaining semisweet chocolate chips, mixing until just combined.
Pour the batter into the prepared pan and smooth the top with a spatula. Tap the pan firmly on the kitchen counter to get rid of any air bubbles. Bake for 25 to 30 minutes. Let the brownies cool completely on a wire rack.
Cut the brownies into squares and enjoy. The brownies will keep in an airtight container at room temperature for up to 3 days.