Preheat the oven to 350ºF (175 °C). Butter a 9x9-inch (23x23 cm) brownie pan. Line the bottom with parchment. Set aside.
In a large bowl, melt together the butter and the chocolate. You can do this over a pan of simmering water or in the microwave at power level 5, pausing from time to time to stir a little so that the mixture heats evenly.
Add the granulated sugar to the bowl of melted chocolate and butter, and stir it in.
Add the eggs, one at a time, mixing well between each addition. Stir in the hot espresso. Dump in the flour, cocoa powder, salt, and baking powder, and stir it in. Fold in walnuts.
Pour the batter into the prepared pan and smooth the surface.
Bake for 30 to 40 minutes, or for however long it takes to achieve your perfect brownies. I like my brownies chewy so I bake them for 40 minutes. Let cool completely in the pan before cutting into squares using a plastic bowl scraper. Don't use a metal knife because it will stick.
Notes
This brownie recipe fits in an 9x9" square pan.
For the dark chocolate, I used Cacao Barry Ocoa 70 % dark chocolate.
Use a metal pan which is a better heat conductor than glass. Brownies baked in a glass pan may take longer to bake.
To cut the brownies easily and cleanly into squares, use a plastic bowl scraper or a plastic knife! Don't use a metal knife because it will stick.