In a small bowl, whisk together flour, baking powder, salt, and cocoa powder. Set aside.
In a separate (larger) bowl, beat butter and sugar until light and fluffy (about 3 minutes)
Beat in egg and vanilla
Mix in dry ingredients (mixture in the small bowl). Mix well until just combined.
Mix in orange juice and zest
Cover bowl in plastic wrap and chill in refrigerator for 30 minutes
Preheat oven at 176C/350F
Line 2 baking trays with parchment paper
Drop dough by the round tablespoon onto prepared baking sheets (3x4 or 4x4). Bake for around 12 minutes (or until it's dry on top).
Cool for 20 minutes