Preheat the oven to 180℃/160℃ fan/350℉/Gas Mark 4. Line an approximately 25cm (10in) cake tin with baking paper.Put the butter and the brown sugar in the prepared cake tin in the oven for 5 minutes to melt and combine. Remove from the oven and give the mixture a gentle stir to distribute.In a large mixing bowl add the flour, ginger, cinnamon, baking powder and salt. Set aside.Arrange the apple slices attractively on the bottom of the pan in the buttery, sugary syrup.In a jug, whisk together the eggs, caster sugar, grated apple and sunflower oil, then mix this into the dry ingredients in the bowl, stirring well to combine. Pour the batter over the apples in the cake tin, then bake for 40 minutes, or until a skewer inserted into the centre of the cake comes out clean.Place the tin on a wire rack to cool the cake for at least 10 minutes, then invert the cake onto a serving plate. Serve warm or at room temperature.