Preheat oven to 180 degrees C. Grease a 20x30cm baking pan.
Mash the brown sugar, butter and molasses together in a bowl with a spoon until the mixture is creamy and thoroughly combined. Mix in the ginger, eggs and vanilla essence.
In a separate bowl combine the plain flour, wholemeal flour, baking powder, bicarb soda and salt. Stir 1/4 of flour mixture into the butter mixture at a time, incorporating each addition before adding the next. Pour the batter into the prepared baking pan.
Bake in the preheated oven until a toothpick inserted into the centre comes out clean, 25 to 30 minutes. Let cool before cutting.