Preheat oven to 180 C / Gas 4. Grease an 20cm square baking dish and set aside.
Stir courgette, apple, eggs, brown sugar, olive oil and vanilla extract together in a bowl.
Sift wholemeal flour, plain flour, cocoa powder, bicarbonate of soda, salt and baking powder together in a separate bowl; gently stir into the courgette and apple mixture until fully incorporated. Spoon cake mixture into prepared baking dish. Sprinkle chocolate chips and sunflower seeds on top.
Bake in preheated oven until a skewer inserted into the centre comes out clean, about 25 minutes. Let cool before slicing.