1. Preheat the oven to 325°F. Butter and flour in a tube pan.
Set aside then sift together the all-purpose flour, lemon pudding mix, salt and baking soda.
2. Cream together the softened butter, granulated sugar and vanilla. Beat for 2-3 minutes until light and pale yellow in colour.
3. Add the eggs one at a time beating well after each addition.
Stop and scrape the sides of the bowl as needed.
4. Lower the speed of the mixer and add the sifted dry ingredients alternately with the sour cream. Mix until fully combined. Stir in the fresh lemon zest by hand.
5. Pour into the prepared tube pan and place into the oven.
6. Bake for 1 hour 20 minutes to 1 hour 30 minutes or until a toothpick inserted into the centre comes back clean.
7. Cool in the pan on a wired rack for 15 minutes then remove the outer ring from the pan and cool completely.
8. To prepare the glaze: Mix together the powdered sugar, lemon juice and melted butter. Drizzle over the cooled cake.