Preheat the oven to 170°C/160°C fan/350°F/gas 4. Grease and dust a 9X5 loaf tin with flour.
Put the sugar, eggs and lemon zest in a large mixing bowl and beat until combined.
Sift the flour, baking powder and salt into a separate bowl and set aside.
Combine milk and vanilla extract in a jug. Add one third of the flour mixture to the sugar mixture and beat well. Then beat in one- third of the milk mixture and beat well. Repeat this process, twice more until everything has been added. 3 additions, ending with flour. Beat until the mixture is light and fluffy.
Pour in the melted butter and beat until well incorporated.
Pour the mixture into the prepared loaf tin and bake in the preheated oven for 75 minutes, or until golden brown, and the sponge bounced back when touched.