Preheat the oven to 325°F. Butter and flour a tube pan. Set aside.
In a medium size mixing bowl, use a whisk to sift together flour, lemon pudding mix, salt and baking soda.
Using an electric mixer on medium-high, cream together butter, granulated sugar and vanilla. Beat for 3 minutes until fluffy and pale yellow in color.
Add eggs one at a time beating well after each addition. Stop and scrape the sides of the bowl periodically.
Lower the speed of the mixer and add the sifted dry ingredients alternately with the sour cream. Mix on low speed until fully combined then increase the speed to medium, beating for 30 seconds or just until fully combined.
Use a large nonstick spatula or spoon to stir in the fresh lemon zest by hand. Pour the batter evenly into the prepared tube pan.
Bake for 1 hour 20-30 minutes or until a toothpick inserted into the center comes back clean.
Cool in the pan on a wired rack for 15 minutes then remove the outer ring from the pan. Cool completely.
To prepare the glaze: In a small bowl, mix together powdered sugar, lemon juice and melted butter. Drizzle over the cooled cake. Alternately, dust with powdered sugar.
Store leftovers in an airtight container at room temperature or chilled.
If you don't have instant lemon pudding mix, add 2 teaspoon lemon extract to the batter along with the vanilla and increase the amount of sugar to 3 cups total.