Preheat oven to 180 degrees. Grease and floor a brownie pan.

Melt butter and sugar till well combined and smooth. Allow to come to room temperature. Transfer this to a mixing bowl. Add one egg at a time and beat well till combined. Add the orange juice and vanilla and beat briefly.

Sift in the dry ingredients into the batter. Mix gently till well combined. Using a wooden spoon, fold in the orange zest and walnuts. Spread batter into pan and bake for 30 minutes till toothpick comes out with crumbs. The top would be golden brown and the edges would pull from the side of the pan.

Remove from the oven and cool on a rack for 1 hour.

Cut into 16 pieces and serve!