Preheat oven to 350°F. Grease and flour two 8 ½ by 4 ½ inch or 6 mini (2 cup capacity) loaf pans. You could also use 1 9x5 inch pan and two mini loaf pans.
Mix the cake mix, pudding mix, eggs, sour cream, lemon juice, milk, oil and lemon zest (if using) in a large bowl and beat for 2 minutes with an electric mixer.
Pour batter into pan and bake. The total time will depend on which size loaf pans you've used. If you poured it all into a 9x5 inch pan, you'll need 60 minutes. If you divided it between two slightly smaller pans, start checking at 45 minutes. For mini loaves, you'll only need around 35 minutes.
Let the cake cool on a wire rack. When completely cool, carefully spoon icing over the top and let it fall down the sides.
Icing: Put the powdered sugar in a large mixing bowl OR in a medium size saucepan. Add 3 tablespoons of the lemon juice and stir vigorously until the mixture is thick, then continue adding lemon juice until is is thin enough to drizzle. If using a saucepan, you can mix it off heat in the saucepan, but warm it a little as you go just to melt it slightly and dissolve lumps so that you can pour it without adding tons more lemon juice.
Spoon over the cake and allow iced cake to sit until icing is firm.