Preheat the oven to 160ºC . Grease and Flour bundt pans or mini-loaf pans. Put the butter in a large mixing bowl and beat until creamy. Slowly add sugar, beating constantly, until the mixture is well blended. Add the eggs one at a time, beating well after each addition. Add the flour and salt and beat until smooth and thoroughly blended. Add the lemon zest and beat another few seconds. Pour the batter into the pan and smooth top with spatula. Bake for about an hour or until a straw comes out clean when inserted into the center. Let cool in the pan 5 minutes before turning onto rack to cool completely.