Pre-heat the oven to 180 C / 160 C fan / gas mark 4 – and grease and line two 20cm round cake tins.
Place the flour, ground ginger, bicarbonate of soda, lemon zest and salt in a bowl, stir to combine.
Place the sugar and oil in a large bowl. Whisk with a balloon whisk, then add the eggs, one at a time, whisking until you have a smooth batter. Now add the parsnip and chopped stem ginger. Stir with a spoon. Now tip the flour mixture in and fold into the batter.
Divide evenly between the two prepared cake tins.
Bake in the pre-heated oven for 25-30 minutes or until a skewer inserted into the centre comes out clean.
Take out of the oven and allow to cool for 5 minutes before removing from the tin. Place on a wire rack. Meanwhile you can make the frosting.
Beat the butter and icing sugar until smooth, light and fluffy. Now add the lemon zest. Drain off any watery liquid from the cream cheese then add to the icing. Whisk until combined and smooth.
Once the cake is completely cool place one of the cakes on a plate. Pipe or spoon the frosting onto it. Place the other cake on top. Dust with icing sugar. Serve.