Preheat oven to 350°F (175°C).
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large bowl, whisk together the cocoa powder, melted butter, sugar, eggs, and vanilla extract until smooth.
Stir in the dry ingredients until just combined. Fold in the chopped walnuts.
Spread the batter into an 8-inch (20 cm) square baking pan lined with parchment paper.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Let cool completely before slicing.