Melt HALF of the chocolate chunks and reserve the other half. Set aside.
In a bowl, whisk together the hot water and cocoa powder. Set aside.
In another bowl, whisk together the oil, sugar and salt. Add the melted chocolate and cocoa mixture and mix until combined.
In a large bowl, whisk together the flour, and baking powder.
Add the flour mixture to the chocolate mixture. Mix until well combined.
Fold in the remaining chocolate chunks and walnuts (if using).
Pour into prepared pan and spread the mixture evenly.
Bake for about 15 minutes, until a toothpick inserted in the middle comes out with a few moist crumbs on it*.
Let it cool COMPLETELY IN THE PAN, about 20 minutes. Then use the overhanging parchment paper to lift the brownies out of the pan. Cut into squares and frost if desired.