Preheat your oven to 350°F (175°C) and prepare an 8×8 inch baking pan with parchment paper or light greasing.

In a large bowl, whisk together flour, cocoa powder, sugar, cinnamon, nutmeg, and salt.

In another bowl, melt the butter and let it cool slightly. Whisk in eggs one at a time followed by the vanilla extract until smooth.

Combine the wet ingredients into the dry mixture until just combined. Gently fold in nuts if using.

Spread the batter evenly in the prepared pan and bake for 25–30 minutes or until a toothpick comes out mostly clean.

Allow cooling for about 10 minutes in the pan before transferring to a wire rack. Cut into squares once cooled completely.