Preheat the oven to 160°C and grease a 9-x-5-inch loaf pan.
Beat the butter, sugar and lemon zest using electric beaters or a mixer fitted with the paddle attachment on medium speed until light and fluffy, about 3 minutes. In a separate bowl, lightly whisk the eggs with a fork and add this to the batter in a few additions, scraping down the bowl in between. Stir the sour cream and vanilla together and beat this into the batter.
In a separate bowl, sift the plain and cake flours and the salt and add this to the batter, mixing until fully incorporated, scraping to the bottom of the bowl to ensure this. Scrape the batter into the prepared pan and tap it a few times to make it level and allow any air bubbles to escape. Bake the cake for 65 to 70 minutes, until a tester inserted in the centre of the cake comes out clean. Remove the cake from the oven and prepare the glaze while the cake is still warm.
For the glaze, heat the lemon juice water and sugar in a small saucepan over medium heat until the sugar has dissolved. Poke holes in the warm pound cake using a bamboo skewer and brush the warm glaze over the cake. Let the cake cool completely in the pan (the glaze will set once the cake is cool) before removing to serve.
The cake will keep well wrapped and unrefrigerated for up to 3 days.